I live in a house with Nu-Heat underfloor heating. It's leaked. Lots. Now the whole lot needs to be replaced.
You can read more detail over at my livejournal, or at least those bits of the problems that happened after livejournal introduced tagging or that I've gone back and tagged. (And some of the problems were from before I started using livejournal at all, like the coils in the cylinders going so those needed replacing to stop them continuously pouring hot water down the overflow pipe (replaced with pressurised ones with an expansion vessel, since they no longer did the old style with atmospheric pressure feed tank on top).) http://www.diyfaq.org.uk/plumbing/plumbingpage1.html#thermal explains the thermal store tank setup.
You can see some other people's problems at http://www.ebuild.co.uk/forums/messages/3322/6281.html
At least in theory the new (FastFlo) pipe material doesn't have the same problem as the rubber hoses they were using ten years ago, and the current warranty terms are better. And Nu-Heat have been generally helpful, and sent repair materials promptly, for free. But if you are thinking of buying a house with an existing system, beware. (Apparently the problem is at least partly caused by copper, so replacing the manifolds and cylinders with stainless steel ones might stop things getting worse if yours isn't leaking yet. I hate to think how much that would cost though.)