I've used the TFO's on my USP, but they're not on there anymore..
IMO, I found them to work ok, but the tritium wasn't terribly bright (maybe they sat in inventory for a long time) and the fiber optic tubes are, by necessity, rather short, limiting their light gathering. Therefore, they worked very well in certain lighting conditions, such as ambient daylight on an overcast day, but would get washed out rather easily on a bright sunny day at high noon or indoors under direct lighting. Also, the quality was so-so, they had casting and machining marks, and required a lot of fitting, to the point of re-working the dovetail angles.
I know there are others out there who swear by them, and if they work great for someone else, cool. But given my eyesight, they just didn't offer the improvement in sight picture I was after.
So I've got XS Big-dots on the USP now, and I'm loving them. Good quality, minimal fitting required. Fresh set too, so the tritium is super bright (Trijicon tubes, BTW). One of my eyesight issues, I've found, is discerning the negative space in a conventional sight picture to get good left/right alignment - this is a non-issue with the XS express style of sights. I can pick up the front without my glasses or contacts. And I've found that they are very quick for 7-10 yard defensive ranges, both in initial sight picture, and for follow-up shots when you re-align. For longer ranges they work too, just takes a bit more lining up.
Some people hate the XS sights, and can't shoot worth a darn with them. But the investmant paid off for me, and my TFOs are in the parts bin now..