UI for non trivial conflict resolution? Definitely.

I dont know about that... Never found they help that much in conflict resolution. They give you some nice buttons for accept their or accept our changes but really I find more often than not those are what breaks code as you often want a mash-up of both sides - which needs to be manually done even in UIs.

Otherwise it is just find the marked sections in the file, and make it look like what you want it to after the merge/rebase. And that is the hardest part - figuring out what it should look like. Which is made easier if you only ever have small commits and merge back to master frequently minimizing the amount your branches drift from each other.