Yeah,the two are totally different worlds, besides being (despite the exclusion of a hi-hat, or any left foot activity) strikingly similar in execution. Creating sound is totally different than mimicking the physical motions that create them. RB is like karaoke where a drill sergeant with an eagle-eye is heckling you. If/when one gets used to it, playing a familiar tune can feel somewhat natural, and I often can get into it physically almost as if I'm playing, but more often it is more of a challenge than something that flows naturally. The good side of that for me was that it forced me to do things I wouldn't have done, even were I to cover the song with a group. Personally, as I'm not and never have been the most disciplined (?) drummer, if I was faced with something I couldn't pull off entirely accurately, I would just do something that sounded similar enough, rather than try to teach myself something new or familiarize myself with something that is awkward to me on the spot. RB definitely stepped my game up in some ways thanks to that gamer score-chasing/perfection mentality. The thing that makes it really annoying in certain songs, is that you know the drummer you're mimicking probably never played this the same exact way twice. Radar Love is a good example of it, twice in one song. All the dancing around on the ride isn't something that was likely deliberately written note-for-note, the drummer is just having a good time. Where one could do something indiscernible to most people while playing that song, making it an EXACT copy can make that a very challenging thing to accomplish. The second time Radar Love showcases the shortcoming of RB drums, is the end where the drummer just starts going nuts and sporadically rolling all over the place while the song breaks down. It's a little too much to be expected to perfectly duplicate someone else's wild and random playing. In my opinion anyway.
The first couple of times I very briefly tried it out at friends' houses when the first RB came out, I was totally lost. Not only because the fact that reading instructions was totally unnatural seeming, but because I'm used to keeping the time as a drummer, not adapting to someone else's, or playing along to a metronome. The other problem, I came to realize later, was that (in the case of the songs I knew, which was all I attempted) my tendency was to play along as I would with the song, which doesn't fly in easy-hard modes. When I finally decided to order a RB2 bundle last summer, I decided that my best approach might be to just start playing at expert, where things would play the way they sound, and it certainly worked out. Made for a rough first week or so with a lot of songs, but most of it came more naturally than when I messed around with other difficulties.
I'm fascinated by how useful of a tool it can be for learning actual drums. I have a friend with no previous drumming experience who plays on expert, and has a pretty respectable record. I had been expressing interest in seeing him try to play drums in the studio for months, and just recently he told me he had went to the studio a few times with some friends. What I didn't see coming at all was that he said he was following along with rock band tracks on the phone to aid in the drumming. While something like that did occur to me, it didn't stop it from sounding really weird. I plan on going with him the next time he and his friends jam, to observe and (hopefully) offer some tips. While I can't call it a drum trainer, the game certainly does have some sort of potential to give people a huge head start.