This is most definetly not the best Spider-Man game, but it is one of the most interesting. While the original Sega Genesis release sold fairly widely (probably due in large part to being one of the only titles available after launch), the later upgraded Sega CD version is an extreme obscurity. The two versions have the same basic underlying gameplay and levels, but are quite different in a number of ways. The original Genesis version was a linear game that guided you from area to area automatically, and the only means of restoring web fluid was to snap pictures of villains (ideally of super villains) and then sell them in between levels to the Daily Bugle. The Sega CD version cut the photo-taking out entirely, and made the game non-linear. You are now presented with a map of New York City at the outset, in which there are numerous locations to visit, though you still have the same overall goal of beating down the various bosses on your way to the final confrontation.
The plot in both cases is ridiculous even by comic book standards : The Kingpin comes on TV and makes outlandish claims about Spider-Man planted a bomb in the city which will go off in 24 hours, and unless he is captured and the location is Gitmoed out of him everyone is gonna die horribly. Apparently just being a rich business guy and buying time on the air is authority enough for the police to issue an A.P.B. for Spider-Man's arrest (at least in the Sega CD version, some faked video footage is shown of Spidey mugging an old lady and webbing her cat as well as hauling a bomb around randomly on his back). The general public of New York seems to have a bit more sense than the police, as instead of panicking and clogging the bridges out of town they just go about their business as usual. Unfortunately for them, the Kingpin actually did plant a bomb (for God knows what reason) that actually will go off in 24 hours. Fortunately for you, that's 24 hours of real time, so you have quite a stretch in which to complete the game - longer than many major RPGs!
While the Genesis version just develops the plot with some stiff cinema of the Kingpin blabbing on TV, and the occasional shot of Spidey pacing around while talking to himself, the Sega CD version has a number of animated cutscenes. These however look like ... well, you ever see that much-joked-on Zelda game "The Wand of Gamelon" for the Phillips CD-I? If you haven't, go Youtube up a video - it looks a lot like that. Short form, it's not too impressive. It looks like something a 12 year old made in Flash.
The Sega CD version also has the dubious "enhancement" of a soundtrack by 80's hair metal band Mr. Big. If you have forgotten Mr. Big, go into any grocery store and wait around until the song "To Be With You" comes up ("So come on baby! Come on over! Let me be the one to show ya!"). That's them. Yeah, seriously, they did the soundtrack. They have one stereotypical 80's movie style classic that blares on the map screen, "Swingtime", which is not to be missed, but the rest of the soundtrack is just instrumental butt-rock in a fairly typical style.
Anyway, comedy value aside, the Sega CD version's non-linearity is the prime point of interest ... but it actually turns out to be more like an NES TMNT non-linearity than a Metroid or Castlevania non-linearity. There's probably about three dozen areas in the game you can travel to, but there's only like a handful that actually contain a boss battle or something that progresses the game's story. The other levels are actually an interesting mix - there's a bunch of moving subway levels around, a number of underground caves and sewers, a battle across rooftops in East Harlem and a brawl out in front of the New York Public Library among others - but there isn't a lot to do in them except to replenish health and web fluid. The only real reason for exploration, other than the sake of exploration itself, is to find hidden comic books scattered all over the game which are then accessible from a gallery off the main menu. This of course actually is enough to keep you poking around for a bit however, just seeing what the game has to offer and finding comics for your collection ... there's also this bizarre pinball game you can stop in on for bonus points.
The Sega CD version did make some improvements over the play control of the Genesis version. Spidey automatically grabs walls and platforms that he's near now, instead of having to tap the jump button again and hope for the best. He also does a much better job of climbing around angles in platforms, though it can still be finicky it's not as impossible as it was in the Genesis game. The Genesis difficulty has been severely toned down - you have more health, can take more hits and there's tons more healing items scattered about the level. The enemies are a bit better balanced and placed in this one too, unlike say the Rapid Fire Hobos in the first level of the Genesis version who were always on a ledge above you firing their endless clips wildly so that you couldn' t jump up and hit them without getting tagged. The boss levels have been redesigned, and bosses can now appear randomly in one of four levels in their "territory" on the map. And some really stupid enemies have been removed, like that forklift driver mini boss in the first level of the Genesis game that there was like almost no way to hit without getting hurt yourself. Bosses are still tough fights, often a little too tough, but the difficulty is nothing like it was in the Genesis version.
The Sega CD version also has a couple new levels, though one barely qualifies as a "level" (it's just one of the usual subway levels but you fight the Vulture at the end of it). There's also Mysterio's Funhouse, and at the game's conclusion Typhoid Mary and Bullseye have been tossed in as sub-bosses before you take on the Kingpin.
* Gameplay Video