First off, you may be familiar with Brilliant's universally acclaimed Bach and Mozart Editions, which emphasize historically informed performances, often on period instruments, and consist mainly of recent recordings by obscure musicians (mostly based in the Netherlands), contracted directly by Brilliant Classics. The Beethoven Edition set is quite different. These are entirely "modern" performances on modern instruments using the standard playing techniques of the post-World War Two period. Most of these recordings were made in the 1950s through early 1990s for other CD labels, and were licensed by Brilliant for this set. There are only a handful of original recordings, and these mainly serve to fill in gaps in the lesser known parts of Beethoven's output, such as the string quintets and the Irish and Welch songs. As a result, there isn't the sort of coherent thematic approach to the Beethoven oeuvre that you get with the Bach and Mozart (and now Haydn) sets.
So how are the recordings? Well, the symphonies are represented by 1974 recordings by Masur and the Leipzig Gewandhaus Orchestra. I was skeptical about these going in, but was pleasantly surprised to hear that they hold up rather well. The Ninth is a perfectly acceptable "modern" rendition of this masterwork. You MUST go out and obtain one of the historically-informed sets by Gardiner or Norrington, since Beethoven's symphonies, like Mozart's, sound best when played on the instruments that Beethoven wrote for, using the ensemble sizes and playing techniques of his time. Among other things, doing this lets you hear the symphonies at the tempos that Beethoven envisioned, something that's often impractical with modern instruments (that have more sound volume and sometimes a longer decay time) and modern playing technique (e.g., habitual vibrato in the string chorus) that evolved to meet the musical and acoustic needs of the late 19th and early 20th Centuries. Once you have that, it's fine to also listen to the Masur performances as credible examples of the modern approach to the Beethoven symphonies.
The Guarneri Quartet handles the string quartets, in recordings from the late 1970s/early 1980s. Since I don't think Beethoven requires seeping sentimentality, I personally dislike their old-fashioned, vibrato-laden, ahistorical playing style, preferring either a historically informed approach, or a more intellectual modern approach such as the Alban Berg Quartet's recordings. But others will enjoy these recordings, and they are technically solid. The violin/piano sonata recordings by Haskil and Grumiaux are artful and remarkably clear for their time, but the sound quality does betray their origins in the 1950s. The unjustly neglected Op. 9 string trios are represented well by the Zurich String Trio, and I enjoy the Borodin Trio and Schiff/Fellner performances of the piano trios and cello/piano sonatas respectively.
Gulda's piano sonatas from the 1960s are serviceable but not very exciting, given the range of quality interpretations available elsewhere, from the classical Brendel, through the cerebral Pollini to various period instrument practitioners. (As an aside, Beethoven piano sonatas are a case where the argument for historically informed performance is less of a slam dunk, since modern pianos are so technologically and acoustically superior to the immature instruments of Beethoven's day.) The Gulda recordings are certainly adequate for filling in gaps if you don't already have a complete set of the 32 "canonical" sonatas (there are three unnumbered sonatas that Beethoven wrote as a teenager). One might have less misgivings about Gulda's recordings of the piano concertos, included here. These seem to hold up better. Sorry, if you want someone like Glenn Gould you'll need to pay real money.
It's great having recordings of both the final version of Fidelio and the original version (when it was still named Leonore). In fact there's also a Fürtwangler historical recording of Fidelio that follows Mahler's practice of inserting the Leonore Overture No. 3 after Floristan's rescue in Act 2. The choral music, including both masses, is generally strong. Of course the vocal and other choral works are included as well as minor works such as dances, wind music, the mandolin/piano sonatas and so on. There are also 15 disks of "historical" performances, some of which are quite interesting, spanning from the 1920s through the 1950s. These duplicate works in the primary set of 85 disks, and there are various works by Bach and Schubert thrown in.
There are some flaws in the set. Several of the Alfred Brendel recordings from the 1960s (chiefly piano variations) are marred by wow or distortion. The soprano singing one of Beethoven's early coronation cantatas clearly can't handle the challenges of the part. Perhaps she shouldn't feel too bad -- these difficulties were sufficient to abort the work's premier. The 27th Piano Sonata (Op. 90 in E minor) is missing its last note.
As with most of the other large "complete" Editions from Brilliant, there are no printed inserts, but you do get a CD-ROM with the program notes. The supplied biography of Beethoven is a pretty serviceable one for its length (about 70 pages), and is worth printing out and reading. The program notes to the recorded works are more uneven. Brilliant Classics is not brilliant in the translation department, so many of the notes read awkwardly, and not every work is covered. There are no English translations to the texts of vocal works at all, though they're usually provided in the original language (German or Italian). Try "The Lied and Art Song Texts Page" for lieder lyrics and translations. For most works, instrumental or otherwise, you'll want to complement the supplied program notes with your own reading [...].
Although most of the recordings in this set are not my favorites of the work in question, it was nevertheless an incredible experience to listen through this man's complete works, something that I chose to do in roughly the order of composition (which required a significant amount of research and planning, since the CDs are organized by ensemble). Although I've been listening to Beethoven music since I was 2 years old, and have studied it for many decades, there were still a few revelations:
1. The three piano quartets that Beethoven wrote when he was about 13. One of them consists of two movements: an allegro with a slow introduction, then a theme and variations, a form that prefigures his last piano sonata, written 37 years later. This Op. 111, in fact, was occasionally used as a two-movement model for other composers, including Prokofiev's Second Symphony and several late works by Webern. I suppose it's worth noting that you'll find antecedents in Mozart too, namely the K. 305 and K. 379 violin/piano sonatas
2. The piano variations in C minor. This composition wasn't given an opus number by Beethoven. Perhaps he himself was unsure whether it really worked. It's basically a chaconne over an eight-bar chord progression that bears an uncanny resemblance to the chord progression used in the last movement of Brahms' Fourth Symphony. The latter is supposed to have been modeled after Bach's Cantata No. 150, but the Beethoven C minor variations sound a lot closer, as their theme is eight bars long (the Bach is four bars), and Beethoven even uses the same chromatic inflection as Brahms in the fifth bar. I would love to know if Brahms was familiar with the C minor variations
3. Speaking of piano variations, did you know that Beethoven wrote variations on Rule, Britannia! and God Save the King/Queen? And have a drinking game, play Beethoven's Auld Lang Syne arrangement for three voices and piano trio, and ask your friends to name the arranger. You'll get few correct guesses, yet Beethoven published dozens of such arrangements during his relatively fallow years of 1812-8
4. For some reason the Opus 1 piano trios had passed me by all these years. The first two may or may not be vintage Beethoven, but No. 3, with its awesome final movement with the modern-sounding axial first theme based on a repeated minor third has become a favorite. Imagine at my age having a new Beethoven masterpiece revealed to me
So in conclusion, for me this set isn't quite the overwhelming artistic and economic success of Brilliant's Mozart, Bach (and even Chopin) complete sets, but for me it was still well worth what I paid to order this from Amazon's France site in September 2007, keeping in mind that as an American I didn't have to pay the sales tax that's factored into the listed price (though the international shipping made up for that). Your mileage may vary, but there's certainly good value here for the price, and I hope that like me you may discover some new music to fall in love with. Brilliant is to be congratulated for putting this music in the hands of those of us who can't spend hundreds to amass it.