I used Mask Pro back when it was version 1. It was pretty good. It did things like trees and hair pretty well. But, it had a little trouble when what you wanted to separate was too similar in color from the background, but there were some decent tools built in to help clean it up. But remember, that was version 1, I'm sure it's been improved a lot by now.
No matter what plugin you go with, it can be a real time saver. However, you should learn how and practice doing it the hard way so you can at least understand what the plugin is doing. Plus, that way, you'll be able to work without the plugin if you have to.
I personally don't use one anymore. I just got used to doing it the hard way. After a while, it doesn't really take that long for most stuff. And doing it yourself, you have more control over the mask and how you get there. A single plugin probably isn't going to give you all the possible ways of obtaining a mask from an image. Doing it manually, you can combine as many different methods as you want.
Also, if you haven't tried it, Photoshop's extract filter can do most of the work that version 1 of Mask Pro did. It can be pretty powerful once you master it, and maybe save you some cash.