Built in exchange function works well... but.. no signature, can't move between folders.
Have tried RoadSync by DataViz- horrible- uses way too much memory and brings computer to halt, and totally lacks any support as it is a beta- unstable and freezes Droid a lot. Used Touchdown- OK, but looks like something from Windows mobile 5 years ago.
The best so far and I really like is Moxier Mail. It really works great. Can move between folders, syncs very fast, integrated in, much more current looking than Touchdown.
Try them all and find the one you like.