I agree the voice actor for the new core is terrible, every time he talks it jolts me out of the game. Haven't tried the co-op so I can't comment on that. I didn't mind replaying the few P2 levels, it was a neat nod to the original game; I thought that zooming past the old Aperture levels you visit in P2 was really cool though. The blue gel-only levels are very boring, it's just covering everything in gel then bouncing for a few minutes before somehow getting to the exit. The levels that give you both gels are a bit better, but every single puzzle that heavily relies on gels is just "make an orange runway and a blue jumppad at the end".
The thing that stood out as particularly bad was the final escape. How bad was it? So bad that the level designers put in an option to skip it altogether. Whereas Portal and Portal 2 slightly push and guide the player so they can play smoothly (mainly the portal funnel effect where the game guides you into a portal as you're falling), the final escape completely lacks this: there's very little room for error, and combined with the way the paint gun behaves (paint spews from the center of the screen instead of from the gun; it also comes out just not fast enough to run on orange gel at full speed) it makes the level really unfun. I spent a good thirty minutes trying to get through it, laughing every time I died because of how bullshit each death was.
Overall the mod might be worth $7, but you might as well download some custom P2 campaigns and have the same amount of fun for free.