experience with Rock Band 3's not-a-keytar, because in many ways it's reflective of your own thoughts on the rhythm genre and pianos in general. Those with actual musical backgrounds are likely to scoff at the easy mode's use of five arbitrary keys for the tune -- not the groupings you see above, but a different set of colors, one per white note in the right half, with indicators just above the notes on that crevice between the top panel and keys themselves (picture's after the break). Even if the actual song calls for a F# note, hitting a F or even a B might be called for instead. It's a mentality that's as old as the genre, that what we're really doing is pantomiming instruments -- or in the case of drums, just hitting key notes and letting the game fill in the blanks on lower difficulties. This, however, is an actual piano. The over-simplification is probably a necessary illusion in standard mode -- or even when the keyboard is used as a replacement guitar / bass -- but having actual proper keys will probably drive the more musically apt insane. Then again, if you're not of that mindset, none of this will probably bother you at all, so feel free to ignore our impressions and bang away.
Pro mode, on the other hand is pretty killer, for all the reasons that standard mode can be irksome. Even at the simplest levels, you're actually getting to play proper notation. Perfect for us, no matter how badly we flubbed up "Crosstown Traffic" on Expert. Sight-reading is a challenge, but nothing worse than actual notation. As we've seen before, the main lane doesn't show all two octaves, but the warning we're given before shifting to the right or left is easy enough to follow, but just like when you first grabbed the plastic axe, expect a good bit of adjustment frustration. See more pics below, and video after the break.
Update: Clarification added to the different color groupings used for Pro and Standard mode.