They're a mid-priced British amp. Generally they sound pretty good to great. They cut some corners on quality - some in ways that matter, some in ways that don't, but actually they have been pretty good at catching problems and upgrading where necessary. General cabinet fit, hardware quality and styling is a bit crude, in particular the quality of the vinyl and the way it's fitted, which is poor (it tends to rip easily and peel off), but which doesn't actually affect performance - although it can make servicing a pain since it tends to catch on the chassis and other parts. I know this sounds like a small point, but if you're a tech it will begin to really annoy you after a while! It also makes the amps look worn and tatty fairly quickly, which can affect the resale value, if that matters.
Of the things that do affect the performance and reliability, they've had well-known problems with valve socket quality, a few issues with transformers, and some other minor ones, but probably a fairly small proportion of the amps. Overall I would put them on about a par with the modern Marshalls, although the failures are more usually to do with parts quality and less to do with design flaws (the opposite of Marshall). And they also cost quite a bit less than Marshalls. Recently they've changed from using particle-board cabinets and own-sourced speakers, to ply cabinets and Celestions, which is a big improvement - check the models of the Celestions though, sometimes they use the more budget ones. The older models with the spring reverbs have a basic solid-state reverb circuit that doesn't sound that good and can fail - the digital ones are probably an improvement.
Hope that helps.
