Ya, it's tough to get on board because I simply haven't enjoyed any of his films that much up to this point, but that isn't necessarily a reason to write him off. He's consistently made movies that are competent, but flawed and not very memorable, which is not what I want out of a new Star Wars.
That being said, as has been pointed out, he may be the right guy for the job on this one given how much he turned Star Trek into a Star Wars movie and as it's well known that he's a fan.
The biggest worries for me then are how he's going to handle the action and how he's going to handle the score.
Up till this point, I've not been a fan of his approach to action, which tends to be way too shaky, cutty, and close-up. I really disliked how he shot the space battle stuff in Star Trek for this reason, the guy seems terrified of Wide establishing shots and almost every Enterprise shot was a medium shot or a close up.
My fear is we'll get a movie that feels decent, but doesn't have any kick-ass memorable sequences in it (kind of like Superhero origin story movies where they save all the good stuff for a sequel).
The juries still out until Star Trek Into Darkness comes out though, because just off the trailer it looks really impressive visually.
Regarding the score, if we're lucky John Williams will still be with us and willing to finish out the new trilogy. If we're going to have all the original cast back, I feel like we need that continuity of bringing back Williams.
If not, the writing is on the wall that it'll be Giachinno. My dislike for his Star Trek score is well documented, but if it has to be him, my hope is that JJ being a big fan of the originals can convince Giachinno to not fuck with the original feel and write something that feels similar in style. If he pulls another Star Trek though, I'm officially done with the guy.