I've used Amplitube X-Gear or that is the one that comes with MC5, I wasn't that impressed with it because it didn't offer many options. I currently use Pod Farm and I'm happy with it. But I will say this, Free or not Amplitube 3 Free is outstanding for a what it offers for free alone, and it's making me take a second look at their software. I think if you buy the entire package it is a bit too high for me, so if I upgrade or purchase any pedals, amps or effects it will have to be only what I need.
That being said, I like the idea of being able to try them and buy a single effect as opposed to having to buy a whole package of stuff you will never use and don't need.
I tried a demo of Guitar Rig before I downloaded this, nothing against Guitar Rig but it took an eternity to install that software, and to be honest, I wasn't that impressed with what it was offering. I guess it's great for some, it's just not for me.