The biggest gain is you get some very useful surround modes that I use almost exclusively. The other so called "gain" is that your getting a receiver that meets or exceeds THX specifications although that usually does not mean that a receiver without it wont do the same.
Yeah, the only tangible thing about THX is the special processing modes. The rest is marketing and testing.
I enjoy the processing modes, though, and my receiver has them. But there are lots of others (Yamaha and Outlaw come to mind) who do not have the THX modes, but are very good receivers, and in many cases have comparable equalization and channel decorrleation sound modes.
I just recently purchased here on the shack my Yamaha RXV465 (great price is why I choose this model) and after going back thru the calibration setting and seeing how much everything was turned down I can't imagine if I turned a blue ray up to "0" as it is. I'm usually at -15 to -20 with a intense blue ray and it scars the dog out of you sometimes. I would love to have a THX rec. though as I too enjoy all the different processing scemes and endless possible settings. Thanks for the replies.