Signing up:tradehill and bitcoin7 ... give me a good reason to use your ref code.
Also, which should I use most (including mtgox)?
Tradhill and Gox are the only ones I have experience with.
TH-R16164 is my referral code for TradeHill. The staff is pretty damn responsive and adjusts things accordingly when they can. The site itself is workable, though needs work done, but honestly, above all, security is going to win. "So far so good" is the only attitude to have for anyone other than Gox in general.
I sadly have very little good to say about Bitcoin7, and I would not tell anyone to not use them as I do not have that personal experience, only experiences with people from the site on the forums and their many many spammers. The way they are dealing with that situation made me not want to do business with that company (basically ignoring any requests to assist in shutting down known spammers, it would be as easy as a post saying Hey, these Referrals No Longer Work
I think having money in Mt Gox is not a bad idea, I think staying in one market is stupid, and even with Bitcoin7's problems, I would give them a shot at the moment just to not have all my eggs in one basket.
Hope that helps.