Well, I have done, and it was a relationship that I have no regrets about, but we'd both known each other a long time and were more or less in love with one another (and I only had a month or so left before I left the country, so it was kind of a 'now or never' kind of thing). I do wish I hadn't hurt a friend in the process though.
Oh, and my current girlfriend. She had a boyfriend but was clearly flirting with me, and despite my complete denseness over the matter for the next few months (partly intentional, since my last relationship had been with somebody who was taken and I didn't have any intention of doing it again), I eventually caught on, and we got together as soon as he dumped her (we later found out he intended to take her back later, although I have no idea what his logic was on that one). That one kind of balances out karma-wise though, since he was an abusive bastard who wasn't fit to so much as stand in the same room as her.
...I was originally going to write "Of course not, unless there were some pretty convincing reasons behind doing so," but then I looked at my answer and realized that it's probably easier to stop justifying it and making excuses and just say "yes."