We always go in Jan., and we love it, but it may not be for everyone.
The temp. varies greatly, but IMO, never too hot or too cold. It ranges from 40 degrees to about 80, sometimes in the same day. In a single week, we often have evenings where we wear hats and gloves as well as swimming weather. This requires bringing lots more clothes (I don't do laundry there). You can't pack seven shorts/tee shirt outfits and a swim suit and be done. You really have to be ready for any weather.
If you are really into water parks, it can be hard to plan for that, since you don't know ahead of time which days will be warm enough. You really can't even plan a couple days ahead, because the forecast always changes day to day. We don't do water parks so it's fine with us.
The park hours are generally shorter, but that doesn't bother us either.
In each of our five January trips, there has always been a major attraction down for refurbishment. TT, Splash, Pooh, Kali, and others. But I can't remember the same ride being down during more than one of our trips. So if we miss it one year, we ride it the next. We always know ahead of time, so we just work around it, but if you don't expect it, it can come as an unwelcome surprise (especially to kids).
We love it, but it may not be everyone's cup of tea.