It's been 48 hours since i upgraded my phone to KitKat and so far, i didn't find any major problem that has been discussed by other people. There were some time when my phone got hotter, but it's still on the acceptable level due to many applications were opened at that time. During normal operations, the phone is still cold as it used to be.
About battery drain, i also have a good experiences. When i upgraded my phone, the battery was around 60% and last night, when i charged it, it was around 15%, so between two days ago and last night, the battery just drop about 45%, which is a good one. During that time, i use the WiFI and doing some activities like browsing, taking pictures, accessing dropbox, and many others. I have to conduct more experiments about this battery drain, but so far, i do have good experiences with it.
Note that i disabled the mobile data and wifi and location/GPS settings most of the time and only activate them when it's needed. I think that gives more battery life to your phone. Also, i tend to close any apps after i don't use it anymore, so it will not hog your resources.