The Playstation 2 version of Rock Band isn't exactly the same as the PS3 and 360 versions. It doesn't feature online multiplayer, DLC, or the character creator. Now, don't get me wrong, it's still the same awesome multiplayer experience as on the other consoles, just slightly different.
Graphics: PS2 Rock Band's background video system differs from the other versions. The background videos on 360 and PS3 use actual polygon characters that you make. PS2's background videos, however, are predetermined CD video clips; a technique that dates back to 1994's Sega CD. Everything looks exactly like the 360 and PS3 versions, at the expense of venue selection and character customization.
Song selection: Again, same songs originally included with the 360 and PS3 versions. DLC was excluded from this.
Gameplay: Despite the limitations, PS2 Rock Band in 4 player mode can be quite the experience. Voice recognition for singing is very well done, and the rectangle notes arn't as hard to hit as some people say. The new drums gameplay is VERY entertaining when you get the hang of it.
Peripherals: The guitar controller that comes with Rock Band is very nice. It's wireless and the strum bar makes no sound! Any guitar hero controller will work, as well as any USB mic (excludes headsets).
Overall, Rock Band for PS2 may not be as complete as the other versions, but it really shines at multiplayer.
If you have at least one guitar hero controller and a USB mic (i.e. the Singstar Kareoke mic), rent Rock Band first to see how you like the interface and song list. If you like it, and you don't mind the limitations, then go ahead and buy the full set!
This would be the best Playstation2 game out there but its not its not even fair because (and sorry Gamefly for spilling the beans) the story mode is way way way different you can get this game if you want its a grate game really but its different.