Very pleased with my BK XLS200. Can't fault the build quality and the sound is a big step forward from a (more expensive) Monitor Audio sub that I was using before (failed and wasn't economic to repair).
I did a fair bit of research into building my own sub on the basis that they're very simple beasts – a good speaker, a good (simple) amp and a well made box is about it. Looking at the cost of the parts there was negigible saving against what BK charge. The same speaker unit appears (rebranded) in subs costing twice the price.
The only drawback is that they lack an auto-on/off circuit. The one on my MA sub wasn't great (it still ate a fair bit of power when supposedly switched off) anyway and i now run the BK off a 'standby saver' type thing on my TV.