Recent convert here...
I had the chance to go UST four years ago when I bought a new Giant NRS1, but decided not to. It was my opinion that there were still a few issues with overall reliability of UST - compatiblity b/w various tyre & rim manufacturers, poor air retention, limited tyre choice etc.
16 Months ago, I again bought a new bike and my research and reading lead me to the opinion that reliability had improved a lot over recent years. So, with much trepidation, I decided to pull the trigger and go with it (Mavic XM819 and Hutchison Spider combo but a conscious decision to NOT use Stans).
At first, I was as nervous as hell at the prospect of having to change a tyre (I had heard / read all the nightmare stories about mounting difficulties) but when the day finally came, I was amazed at how easy it was to do (although I think that is a reflection of the good compatibility b/w the rims and the tyres).
Like I mentioned, I don't use Stans as I didn't want to deal with the mess. As luck would have it, I got my first UST puncture last Sunday. As per the guidance on the Hutchison web-site, I simply folded the tyre at the point of the puncture and applied a small dab of superglue to the "wound". Five minutes later, I reinflated the tyre (beads hadn't un-seated so my small hand pump did the job) and I was off and running - too easy. This sealed (no pun intened) my faith and I am now an avid convert.
That said, I still carry a spare tube in case of a major tear.
Cheers,
Sam.