PRO Guitar: This 122 button controller with six strings is intimidating to look at, as it should be. Guitar is a fairly compllicated instrument to learn and taking each song part by part and learning how to play it is very satisfying. I can attest that I took a week of learning "Ring of Fire" by Johnny Cash and could play it fairly recognizably on a real guitar.
PRO keys: Don't even bother with regular keys. Start with PRO. They keep it fairly simple on the easier parts until you amp up the difficulty. Eventually you are playing the part 1-to-1 and can take that same skill over to a real piano and play the right hand part of the song. I also verified that I could play the simple keys part of "Werewolves of London" on an actual piano.
PRO drums: I haven't hopped on a real kit yet but the cymbals that you can attach to your kit are AMAZING. Drums went from my least favorite to my absolute favorite instrument to play thanks to these discs of awesomeness.
Harmonies: Download a couple duets and see how much fun something like "Paradise By The Dashboard Light" is with some friends. The harmonies add a great extra participation layer to parties and those who want to voxtar or voxkeys (play the instrument and sing).
Visuals are great with some odd animation transitions, the soundtrack on the disc has a great starter variety, the DLC is unmatched and incredible. RB3 is a gateway and almost like a client to get you expanding your library with great songs from all over the musical spectrum.
If you like music, if you like social games, or if you like the idea of living out a rock-and-roll fantasy then Rock Band 3 is the ultimate experience.