I used to think "it's possible they exist, but not probable." But then I had a very close friend tell me about his encounter and I started reading more about the subject. This friend is very intelligent and analytical, and isn't prone to jump to conclusions (you know, the guy that will ALWAYS reply with the snopes article disproving that email forward that just got sent out). I was going to write the Coles notes version, but then I remembered it was reported here: http://bfro.net/GDB/show_report.asp?id=1400.
I decided to give it a fair shake and have since read a few good books on the subject. The more I read, the more I'm convinced. Laugh if you must, but all I have to say is, do some honest research and I think most people would be surprised about the amount of evidence there is for their existence (people have been convicted of murder and thrown in jail for less)!