I pre-ordered this, and was really excited for it, but after a night of playing, I'm less than thrilled.
If you want to play it like a keyboard rather than sling it over your shoulder like a keytar, it's hard to find a surface to play it that won't make your hand hurt after a couple of songs. (Maybe the Mad Catz stand would help? I wanted to try the keyboard out before deciding whether to get the stand, and based on my experience so far, I doubt I will be playing it enough to make the stand worthwhile.)
There were no real instructions that I could find, in-game or in the accompanying documentation. It didn't take long to figure out through trial and error, but failing the first few bars of a song while you figure out what buttons do what is not a great user experience. The interface is not very intuitive (like the rest of the over-shell in RB3), and some basic "I see you've plugged in a keyboard for the first time. Do you want a quick tutorial?" in-game would have been very welcome.
I play a little piano, so I expected to have a head start on pro mode. To some extent that was true, in terms of knowing my way around the keyboard without having to look at my fingers, but the way the notes are represented in pro mode was really frustrating. There's an on-screen "track" for every single note, and I found it really difficult to tell which notes it wanted me to play.
If RB3 had an option to switch to standard sheet music, I would have taken it-- had I spent the same amount of time at my piano with the sheet music to "Imagine" as I spent trying to figure out which notes Rock Band wanted me to hit in that stupid bridge, I would be able to play the song from memory today.
The regular non-pro mode seemed a little silly: since you must play it one-handed, it does not have the same level of involvement as guitar, bass or drums.
I went through three of the pro-keyboard lessons, but the software kept glitching to the point that it was unplayable and I had to reset the console. Not impressed.
And, although you couldn't call it a surprise, if you have imported your RB2 tracks or have a lot of DLC, there are just not that many songs that have keyboard parts to play to begin with. On tracks without a keyboard part, it gives you the option of playing guitar or bass parts on the keys, which I tried, but found that I would rather be playing the guitar part on the guitar.
In short, I am not really eager to rush home to a second night with that keyboard-- which is a shame, because I'd been counting down the days until it was delivered. Oh well. I still love my drums best!