Aside from the unfinished Zones (Wood & Hidden Palace), some of the existing levels (namely Metropolis and Mystic Cave [known here as "Dust Hill"]) have drastically different level-design from the completed version. There's even a few Badniks that didn't make the final cut, so there's stuff to see all over the place. This is the most worthwhile download I've come across since those Sonic Adventure movies...
The easiest way to access all the levels is to use the Sonic 1 level-select code at the title screen (up, down, left, right, A+START). Everything's very glitchy and none of the Zones are complete, but that's to be expected. For more info, check out the Sonic 2 page in the Sonic Museum. Have fun!