I have a ton of Milwaukee tools, including most of the V28 cordless line, several of the M12's, a corded drill, and the 12 volt cordless version of the caulk gun.
I haven't had a single failure on any of their tools in the last 15 years.
I think the weak point on milwaukee tools are the batteries. Nothing compares to Panasonic cordless tool batteries.
Now about the caulk gun. I think 18V for one is pretty much overkill. My 12V will empty a conventional tube of caulk in no time at all. It has a pretty good range of speeds, so it's not to hard to vary the bead size.
If you are doing a lot of caulking, it is a wrist-saver - it is significantly heavier than the manual caulk gun, so that takes away from the benefit somewhat. You can buy the big tube carriage shown above, but I never bought one.