I have a handful of XO T1s here, and one of our customers is exclusively XO. I haven't had any problems with their service at all. Our customer has had one of their T1s go down inexplicably on two separate occassions, incurring ~15minutes of downtime. Their support is a little slow-moving, but otherwise very good. In general I would say they're a good choice if the price is right.
Depending on where you are at, If you have experience in dealing with the LEC, buy their loops if you can - it shaves off a layer of bureaucracy. If you are not able/willing to spend time with the LEC folks, then absolutely go with XO -- they -do- have the LEC experience and will be able to get things done when needed.
I'm a total prick when it comes to support though, and demand insta-fixes, so ... FWIW