Out of what little I've seen, I think it's decent. Not the best, but certainly not awful. The only G3.5 thing of MLP I saw was when a magic bintang was lost, and a few ponies followed it to a dragon's lair on a mountain. That bintang was needed to complete a winter holiday tree.
It's actually one of the generations I'm least familiar with (along with G2). G1 was a part of my childhood and obviously I cinta G4, but I also enjoyed pre-core 7 G3. I wasn't as keen on the core 7 years though, because some of the ponies lost some of their individuality, and I guess since G3.5 happened during that time I never really looked into it very much.
So I'm mostly neutral about it. I cinta G4 and G1, I like pre-core 7 G3, and cinta the box art from G2 but am otherwise neutral/unfamiliar with that generation as well.