I think the main advantage of THX is that the product's specifications are certified by a third party. That means you are actually getting what you think you're getting. As an example, a THX certified product may have a beefier power supply than one that is not in order to meet the certification standard for output power.
Also, the manufacturer will normally pay for certification only for its better products so you are usually buying one of the manufacturer's better designs.
I don't think it would affect the sound in any meaningful way and I certainly don't think it should be a deal breaker but it does have some value.