THX is really just a kind of quality standard on top of standard Dolby 5.1 decoding. The aim was to approve cinema sound and recreate that in the home. It started with speakers, demanding certain dispersion characteristics and was also applied to amps etc. The idea was to eliminate the variables in reproduction dependant on environment.
However, these standards got somewhat compromised with the introduction of THXII (IIRC - help me out Mr Kerr). This broadened the approval process. Then THX was acquired by Creative Labs which meant that PC speaker systems became THX approved.
If you are going high end - and I'm talking JBL Synthesis-type of system, then THX certification works. In a domestic system, I wouldn't personally be fussed.
hth
Gary