I'm an Infiltrator, I've tried other classes and I just don't care for them as much. I've learned to rely so heavily on Tactical Cloak that I can't deal without it.
Seriously, it's such a life saver, and it makes lining up sniper shots a lot easier.
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~Halo Rolling around at the speed of sound.
http://img593.imageshack.us/img593/2417/sonicspeed.jpg