I'm currently only at world 5, but I've seen most of the rest of the game from videos. I'll make you a pros and cons list:
Pros:
Lots of original things programmed. This ranges from things Solrgryn uses a lot like double jump replenishers, which he has found various creative ways of using, to one-time threats like rainbowish Wario chasing after you.
Creative and funny traps, the ear-rape spider in particular.
Excellent® level design that often makes great use of the specific elements of that world's theme. For example, that segment in VVVVVV where you actually have to avoid instead of use the down arrow things. Not to mention that this was an example of a creative use of the double jump replinishers. I hear you think this game has bad level design. I want to hear you explain where and why.
Lots of well-made references to video games. Many fangames either lack these or fail to implement them well. For example, VVVVVV world, which cleverly uses the different color people to switch between normal and VVVVVV gravity.
Great choices in music. Can't really explain that objectively.
There's a hard-to-explain sense of epicness in the boss battles. It probably has to with the screen-shaking (which I don't actually mind), the fact that bosses actually have animations, and the wide range of well-programmed attacks.
Plenty of optional achievements that add replay value and give you something to work for besides winning the game.
It's kind of fun being able to pick and try out the various characters. I find some of their shooting animations kind of funny and clever, like Sonic shooting rings.
Hearing LOL U Died everytime you die sometimes takes away from the frustration. In particular, if after finally succeeding at a tough segment, you shoot a save point and get killed by its spit attack, listening to LOL U Died serves as a sort of celebration.
I thought that save-jumping seemed like a clever yet challenging use of a mechanic that's existed since the original IWBTG. Too bad he took most of it out in recent versions of the game. I wanted to try it out.
Cons:
Solgryn is often a dick and makes us do annoying things after hard segments before we get the save point. The examples I've so far faced are the Mario dance and the blind jump after the Gravitron. Both of those took me 3 times and really pissed me off.
Yeah, that's all I could think of for cons.