I would say that the game gives you a decent enough foundation, and, on Expert level, is a good way to practice and gain proficiency if you have drummed before and have some basic technique down. Playing along to your favorite songs is a time-honored method of practice for most of the rock drummers I know out there.
There are, however, several important things Rock Band does not teach you. Left foot proficiency, for one. Those open high hats are usually hit on the blue pad (moving from the yellow to the blue), but in real life you’d still be hitting the same “pad” (the hi-hat, in this case), while lifting your foot.
Another important thing to note is that dynamics is not taken into account in the game. A lot of the musicality of drumming is not just putting the beats in the right place, but how you hit them. Techniques involving stick bounce, I’ve found, are a pain in the ass to get consistently right in rock band, so sometimes I’ll find myself hitting the snare pad (red) with a RL sticking pattern, when I know the drummer is really just doing a double bounce with the left hand on the snare.
Also, there’s a whole range of sounds you can get on different drums and cymbals depending on where you hit them and how you hit them (regular hits, scrapes, catches, etc.)
That said, however, I do think it’s a good way to get started into drums and it develops coordination and rhythm, as well as ways to really understand and try different drum parts in different styles that you may be used to. I’m just a wannabe drummer myself, but expert mode on Rock Band really has improved my real-life drumming a lot, and has given me lots of new ideas and exposed me to different beats I would not normally be playing along with. The drums on Rock Band, in expert, do translate to some real world skill. There’s a lot more to drumming than what happens in Rock Band, but it’s a good start.