Okay, time to throw in my two cents...
Being one of the people who played through on co-op with jake_heritagu, I'll try not to repeat everything he said: partly because I don't need to repeat all of his points, and partly because my opinion differs to an extent.
First off, I love the looks (mostly). The new textures and palette is really great, and I love the new objects. I really loved the 'wavy' water effect and how horizontal doors can work now (not sure how they work yet, but I know I saw some in CEZ). I like the concepts for the enemies, but some of the sprites, not so much. Really didn't like the Crawlas. They looked like someone took Senku's crawla sprites and made them look not as good. Then again, this is my personal opinion. Oh, and that while some enemies got redesigned the Jettys and Buzz...I always forget if it's just Buzzes or Buzzers, but I assume you'll know what I mean by that, but the sprites of those two enemies didn't look touched.
As for what I didn't like... well, level design mostly. DSZ I felt was too long, and the underwater parts got on my nerves. Granted there were air bubbles, that sort of makes it feel slow paced, especially when you're playing as Sonic. I did not like the diverging path in ACZ. That got me and my friends lost, because when we first saw that pit, we figured it was one of the deathpits that were common throughout the level. That might be our own mistake, but the length of the other path had us convinced it must be the right direction, when it wasn't. It didn't get better after that. In either case, I felt like I was being punished for being a Sonic. I had to stop many times to find the right path, and I felt for a speed-based character I had to stop running quite a lot. It seems like if you go Knuckles, you'll have probably the easiest time getting through the 'new' SRB2.
All in all, whether you choose to agree with this opinion or not, I feel like I'll be sticking to 1.09.4, the last good version of SRB2, once again, in my opinion. That, or I'll have to rip most of the new stuff and put it into a mod for 1.09.4, so I can enjoy both the old and the new.
(Oh, also, not going to comment on multiplayer because I never do it ; shooting rings at other people? Seems so silly.)