I am just reviewing the main extra track here, as all the other reviews tell you what you need to know about the main album.
I got this as a 2-4-1-pack (Eldorado plus Secret Messages)in a clever but cheap cardboard sliding (aka scratching) sleeve, but the CDs inside are gems.
I bought the Eldorado extended edition because I had always wanted the Instrumental Medley bonus track. It's exactly what I had expected - some of the orchestral / band 'backing tracks' (no voices) all joined up into an 8-minute mini-overture - just like you're at the opera! It's def worth getting for this extra. Ideally, it would be at the start of the album, to settle you down ready for the main album, overture-style. (You can even have an 8-minute karaoke session on your way to/from work.)
I always thought of the album as a mini-pantomime soundtrack, with the whole recording being quite 'close' - you are in a small local theatre rather than a massive concert hall as the later albums. My vinyl album was very fuzzy, but this remaster is excellent - all the orchestral instruments loud and clear, adding to the magical 'school orchestra and local opera group/drama group' effect. All the ideas that came to fruition a little later (as other reviews) are in place, but with the production a tad under budget. But this is part of the charm. There is a strange magic ;-) to this whole album - a Christmas feel to it. I give it 4 stars because there is no 4.5; and New World Record probably pips it, as nothing will top the last 3 minutes of that one - and there are plenty of other ELO albums just as good or better. The triple box (or, rather, 'book') set is great, too, if still available.
The 2nd extra bonus track is a throwaway 45 second add-on.
I hope that helps if you're looking to buy the extended edition.