In my personal opinion, I'm okay with rainbow ponies, though I must admit, I cannot admire the designs for too long since it does hurt my eyes.
However, the designs for the new rainbow-fied "equestria girls" does not sit well with me...the designs just look hideous on the human versions; they seem like they are a rip-off of "Jem and the Holograms", which I also found the colorful girls disturbing in looks.
Another personal opinion: designs aside, I didn't even like the first "equestria girls" movie. It just makes me wonder why there would be a sequel since the first movie, before it was released in theatres, gained a lot of negative attention. Or at least from what I saw on blogs and other online communities, the general opinion was a dislike of human versions of the mane six. But again, people gave it a chance and from the sounds of it, most of them liked the first movie. In that case, I think the advertising was really poor as it is with the sequel, whether people like the movies or not.