My partner has been with Triple Canopy for nearly 2 years now and only has good things to say about them. As for Blackwater, Aegis, Dyncorps, his experience in working around them has been..mixed.
You don't necessarily have to be x specops, or even x anything to get in at Blackwater. I had a 20 year old guy with zero experience who was in decent shape on one of my courses here with globerisk.com, and he was so horrible with basic small arms he was becoming a danger to himself and others, so we washed him out on day 2. Well, he begged and pleaded to stay, so after spending 2 days of 1 on 1 training with him, he began to catch on somewhat resonably. Fast forward to today, and he's been with Blackwater for nearly a year now. Go figure.