Personally I would never do this, but a friend of mine is looking to do so, which I'm trying to discourage. You of course pay a premium, which is understandable, which saves the significant time investment of building yourself, but you gain zero knowledge of the process, which is certainly an issue when it comes to maintenance. Clearly there is a big market for pre-built WC systems though, but it got me wondering about warranty... obviously individual components are covered, but what about the build itself, and if over time you suffer a leak and components get damaged? How would you prove it wasn't user error messing about with something, a fault in the original assembly or a specific fitting fault for example? And what kind of compensation are you entitled to if it were one of the latter two?