Ooo-aahh ... do we have to set up secret identities to answer this one?
One of the thoughts I have had on this subject is that by sending out to multiple at once you can freely say that the manuscript has not yet been rejected by anyone else and be totally truthful. Doing it serially doesn't leave you with that freedom.
I can understand agents objecting if you have already sent the manuscript to publishers. I can't see a problem with sending to multiple agents at once. (Assuming you can find multiple - in Australia there is a real dearth of publishers/agents that will accept fantasy novels from unknown authors.)
ETA: To actually answer the question: No, I haven't. But perhaps due to lack of choice rather than lack of temptation.