I recall having a great time with these types of games with friends. They were kind of like karaoke without the singing part. The later editions with more options for setting difficultly per player (IIRC) made it even more fun since you could have some people who were more experienced being given more of a challenge while a newbie or less coordinated person could play at a lower difficulty level and still have fun.
The room full of crap that sat around was not fantastic, though. We live in a smaller house at one point, had the drum setup and a couple guitars. The drums were hard to store, got in the way and just sucked except when you were using them.
I'd probably be interested in picking up something like this if I could get a controller that would work with any arbitrary game, as I'm going to guess that there are going to be fun songs on both games. My kids always loved the guitars and they got some appreciation for non-kid music since the track selections were pretty decent. I'd also appreciate if they would bring the songs from earlier games forward, too.
In terms of being agnostic, it would be nice to bring your fake guitar to your friend's house and play whatever they had, irrespective of if it were Rock Band/Guitar Hero or Playstation/XBox. I'd bet overall they could move more copies rather than try and keep it siloed. I'd hope Activision would see at least part of that with their success with Skylanders -- you don't have skylanders for each platform, you can take them to your friend's house and play on any console.