I'll be getting my 8G any day now. I'll let you know if it works or not. It should be compatible though. There are lots of people on here that claim to have them.
I have had a 8gb Seagate micro drive for over a year not with not a single issue. My X51V recognized it as soon as I inserted it into the Cf slot. I have a 6-1 card reader and it was recognized by my pc just as fast. If you have a CF card reader I would see if your drive can be recognized by your pc through the card reader. If it does and your X5 then your X51V may have a issue with its CF slot. I would also see if I could borrow some ones CF card and see if it can be recognized by your X51V. By doing all of this and recording the results when you call for a replacement it will cut down on all of the standard things they as you to do prior to sending you a newely refurbished unit.