I know from my experience and talking to other folks in recovery that not every “sex therapist” is really qualified to deal with this complex addiction. In fact, early on I spent six months with one Ph.D. who didn’t get it, and it didn’t do much good. (He was a really nice guy, though.) At about nine months into recovery, I chose to enter treatment for five weeks. My therapist was qualified, had a good handle on the disease, and saw through my b. s. in about two minutes flat. I continued to see him privately for nearly two years. He was a huge factor in my recovery.

A good place to start.
The first thing I would look for if I chose therapy again (with someone else) would be someone with a “CSAT” (Certified Sex Addiction Therapist). The CSAT training program is pretty much the standard, and every CSAT I know is knowledgeable and up to date when it comes to this addiction that strikes to the core of who we are. Many are in recovery themselves and know whereof they speak, inside and out.
When it comes to partners, a CSAT is also the best bet. We had good luck seeing the same therapist, alone and conjointly. If there are trust concerns or worry that the other party’s therapist may be biased, consider seeing him or her individually before going together, to get a feel for the professional outlook. A real pro will not take sides. If it’s clear that one is doing so, consider finding another certified practitioner for both parties.
That’s about it. Not a lot of information, but it would have been useful to me in early recovery, so I’m passing it on for whatever it’s worth.