When people talk about Sonic making the jump to 3D, I'm willing to bet a lot of people usually don't talk about this game, and for good reason. Bar from the fact that it's not a true 3D game, it's just not a great Sonic game altogether.
The story of Sonic 3D Blast follows the same rough formula the other Sonic titles do. Robotnik turns innocent animals into robots, Sonic must go and stop him. There's a little bit more to it than that as the game's opening explains; Robotnik discovers an island where mysterious birds named Flickies live. They have the power to dimension jump through giant rings, and Robotnik wants that power for himself. So he kidnaps them and forces them into robots. Sonic just so happens to be friends with the Flickies, so when he comes to visit them, he just sees robots everywhere so decides to rescue his friends.
The gameplay though, is very different to what you'd expect from a Sonic game. Granted, there are a lot of familiar things; Rings help you survive hits so long as you're holding at least one, TV monitors offer power ups to help you out, etcetera, etcetera. Those are the only things, however, as Sonic 3D Blast is an isometric platformer. I feel that the gameplay here is clumsy and awkward; I found it so much harder to be precise when it came to making jumps across smaller platforms or hitting a Badnik without taking damage myself. Not only that, but this is a much slower paced game. I know Sonic's not all about speed, but it still doesn't appeal all that much to me.
The thing that slows the pace of the game down is the Flickies. In this game, you not only have to release the Flickies from the Badniks, you need to pick them up and lead them to the nearest Giant Ring. Each level is broken up into sections, with usually around 2 or 3 sections in a level. Much like other Sonic games, there are two acts to a zone, and in this particular game, there are 7 zones in total. In each section of a level, there are five Flickies to rescue. Release them, catch them, take them to the section's Giant Ring. Once you've rescued all five Flickies in that section, you can move onto the next section. Lather, rinse, repeat, until you beat the level. Now in all fairness, I can see why the game simply doesn't have you get to the end of each level, because the levels are pretty short otherwise; rescuing the Flickies is purely to drag the length of the level out, but it doesn't mean it's acceptable.
There are four different types of Flicky you can find; blue and pink flickies make an effort to find you; these guys are pretty nice and make your life easier. Green flickies and red flickies aren't anywhere near as helpful, however. The green ones will wander aimlessly, sometimes even trying to get away from you and the red ones bounce around all over the place; I find them to be the most annoying ones to grab. The only time you won't be finding and rescuing Flickies is the boss acts, where you'll be fighting Robotnik. I find these bosses to be relatively easy, at least in terms of strategy, but I still find myself getting hit a lot, mostly due to the camera angle.
Throughout each act, you can also find Sonic's good friends, Miles Tails Prower the fox and Knuckles the Echidna. When you approach them, they'll take Rings from you and once you've given them a total of fifty, they'll whisk you off to a special stage. The Mega Drive version lets you do this in every act of the game, meaning you have a lot of opportunities to grab all the emeralds. The special stages are insanely easy. Much like in Sonic 2, the goal is to collect a set number of Rings and avoid the bombs. Unlike Sonic 2, however, the special stages rarely switch things up in terms of patterns, the bombs are easy to spot and avoid, and more often than not, I wind up with around 100 Rings over the quota - which also never changes by the way. Collecting the Chaos Emeralds unlocks the true final boss of the game, nothing more.
The graphics aren't impressive on Mega Drive, it actually looks quite blocky. Though while my eyes aren't particularly appealed by the game, I can at least listen to some pretty good music. I really like a lot of the tracks in this game, and they must have gone over well because they were remixed in later Sonic games, most notably Sonic Adventure.
Overall, I found Sonic 3D Blast to be a mediocre experience. Not a good one, but not an outright terrible one. There were a couple of points where I had fun, but most of the time I spent playing I couldn't help but feel that I would rather be playing other games. I just don't think the isometric approach works for Sonic all that well, and that coupled with the tedium of hunting down Flickies just drags the game down. If you have to play 3D Blast, I'd recommend trying to track down the Saturn version as it irons out a few of the issues existant in the Mega Drive version and has better special stages to boot.