The game's interface shows incredible fidelity to the console version, right down to menus, sound effects, and a look and feel that closely mimics the original. Without plastic instruments, though, gameplay is of course fairly different, as you're pressing touch-screen buttons in time with approaching notes on bass, guitar, drums, and vocals--but the game still does a good job of keeping a Rock Band feel, with the same scoring, stars, and crowd, streak, and energy meters. You even tilt your device for "Overdrive." The only weaknesses are a less than satisfying vocal system (no microphone, you're just tapping more buttons) and the fact that the interface can feel kind of crowded at higher difficulties (no problem, though, if you're not very good).
Understandably but not surprisingly, the biggest drawback of Rock Band Free is that you only have access to one song, in the game's "Quick Play" mode: Foo Fighters' "Learn to Fly." For people who really, really, really love this song, Rock Band Free has excellent replay value.