Mind-blowing animation quality + classic Street Fighter gameplay + tweaks and Parry system = Superb 2D Fighter
GRAHPICS:
On the technical side, Street Fighter III is incredible... Each character has an incredible number of frames for almost all possible animations, everything in this game looks super-slick to the point where it puts B-grade anime to shame and gives the A-list quality a run for its money. While not quite up to snuff with, say, Kill Bill's sequence or Samurai Champloo, it's far and away better in framerate than action shows such as DragonBall Z, Inuyasha, and Ruroni Kenshin. Levels aren't as vibrant as something out of Street FIghter II or the Alpha series but I believe this more ties into the artistic style of the game.
Stylistically the game is somewhat less impressive depending on your character. All of the characters have a much more mature sort of appearance. Classics (the Shotos and Chun-Li) look incredible in this game, seeing them move so smoothly is complimented by the new good looks of the actual characters. The designs haven't drastically changed, but they are noticably different and in my opinion it's for the better.
Some of SFIII's characters feel out of place. While SF2 and SFA were a bit realistic, this cast is just kinda not. And like many have said before, it feels like something more at home in the Vampire Chronicles/Darkstalkers series. Shapeshifters, robot assassins, a guy apparentley made out of silly putty to subsitute Dhalsim, etc. just don't have the same charm as the older cast does. Some of the other new characters don't suffer from this though. In particular my favorites are karate practitioner Makoto, ninja Ibuki, British boxer Dudley, and the whatever-the-heck they are pair of Yun and Yang.
The backgrounds suffer the same problem in that they range from 'yeah that makes sense for a Street Fighter game' to 'Wtf did they pull this out of!?'. Generally it suffers much less though, there are only a few odd stages.
SOUND:
Quality-wise, all the sounds are crystal-clear and drive their point home. Nothing really to discuss as audio hasn't really got much more room to move unless you get into surround sound (which makes no sense for a 2D fighter).
All of the background music has a unified theme now, as opposed to SF2's distinct styles for each level. Not necessarily a bad thing, however. They're all sort of modern-feeling hip-hop/jazz tracks, and they move the action along nicely. Not a lot of them are stand out great but a few do get pretty catchy.
Character voices seem pretty spot-on, but then this term kind of is meaningless against some of the more odd new characters. Regardless none of them sound out-of-place or as bad as the old SF2 voices did.
GAMEPLAY:
Unless you slept through the 90s, you're probably aware Street Fighter II blew up. Huge. There's good reason, the game's a tremendous amount of fun. Rather than go over all the basics of SFII I'll just go over what's new in SFIII:3rd -
High jump/super jump - If you've played most of the Vs. games you know what this is about. Pressing down shortly before you attempt a jump makes you go higher/further depending on the character.
Dash - Probably also familiar by now, double tapping in a direction makes you move at an increased speed in that direction for a little bit, and also leaves you vulnerable.
Roll - Tap down as you land, character rolls to safety. Speeds up matches a bit
Universal Overhead - Pressing MP + MK makes every character jump up and preform an attack that can only be blocked standing up. Adds some mix-up options
Parry/Red Parry - What makes III such a massive improvement. Parrying is a defensive manuver that negates all damage (no chip) and assuming you're not parrying a projectile, gives your opponnent a bit of a freeze. This opens up a lot of counters and opprotunities for supers. Red parry is like a normal parry, except you perform it after blocking one move of a successive attack (autocombo moves like supers, rapid-fire punches, etc.). Its function is the same as a normal parry.
Also, before you go into a match, after you choose your character, you have to pick one Super Art out of three. And with the exception of Gouki/Akuma, that's the only super art (super art = new name for super combo/hyper combo) you'll be able to perform.
The game sports some other nifty features, like an in-depth training mode with advanced features like dummy recording which let you perform moves as the dummy and then go back as the character you picked and face them; an incredibly lengthy option menu which lets you tweak a lot of features and change some of the fundamentals of the game, and can, for instance, disable parrying, jumping, blocking, enable air blocking, allow you to use all three of your character's supers as opposed to picking one, etc... The pages for this menu unlocks as you play through the game without any of these said tweaks. You can also save replays in Vs. mode, there's a grading system that judges your performance after every match, there are updated bonus stages, certain moves can be powered up at the cost of Super bar, blah blah blah blah blah. There's a LOT of substance here.
With that said this game is as replayable as SF2, if not more, for some of the unlockables and added features it gives. Naturally Vs. mode is where it's at and will keep you occupied well into and after the release of the hypothetical Street Fighter IV.
My review can be summed up like this: If you enjoy SFII, get SFIII: 3rd. It's leauges above 'worth it'.