Pretty good! For a long time, HTC was an OEM manufacturer - they supplied hardware that was rebranded, marketed and resolf by AT&T, Orange and similar folk. More recently, they starting supplying phones under their own brand, similar to Nokia or Apple.
Currently, most HTCs run Windows Mobile - if you like this operating system, HTC phones are probably your best bet. The T-Mobile G1 and G2 are also HTC phones - they both run Android.
Generally, the hardware is pretty solid, they're well built, and have been reliable enough for carriers to resell them for years.