I really only play at 32Red. I mean >90% of the time. Now and then I deposit $10-$100 in some others, but whenever I've got a larger wad of e-cash (not very often) I ALWAYS deposit at 32Red. It's a matter of trust. My reasons:
1. I trust them. I believe that my money is safe there (apart from me gambling and losing it all of course ). I know that if I should win $10,000 (yeah, right), I wouldn't need to lose sleep over whether I'd be paid or not.
2. Their customer support is top-notch. I always get a friendly response to my e-mail queries within 15-30 minutes. I think they've become very popular these days, it used to be less than 15 minutes. Anyway, 30 minutes is OK in my book. They also have an instant chat option, always respond within a couple of minutes, and you can ask them to credit a bonus instantly or flush a cash-in or whatever, they'll do it right away. IMO, a working INSTANT CHAT system is extremely important.
3. I have never read a legitimate complaint about 32Red anywhere.