I'm at Pax now doing OT, but am friends with and work with several TPS grads. Also, I've got done some DT assist/IT with HX-21.
Here's the basic run down:
-If you're selected you to a TPS program. Most guys go to Naval TPS at Pax River, but as others mentioned there are some exchange programs with Britian's Empire Test School and France's program. I think pointy nosed guys can also go to AF TPS. USNTPS is pretty well recognized as being an incredibly rigorous academic program. My understanding is that the folks who go to ETPS come back loving life while USNTPS grads take a few years to recover from their beatings. You do get the opportunity to fly a lot of different aircraft. TPS always has all sorts of cool aircraft in the air. I've seen MiG-15s, B-25s, and T-28s in the past year. As a RW guy you'll get a chance to fly gliders and pointy nosed aircraft such as the T-38 and F/A-18 and a whole mess of helos: H-57s, H-58s, H-72s, and H-60s. All of this flying is not just for grins though, you're out collecting data so you can write and be graded on reports. And there's a lot of writing in the test world. Test plans, yellow sheets, and test reports to name a few.
-Following TPS you go to one of the test squadrons for a few years and conduct different tests. As a DT guy you're certified to be able to conduct tests that fall outside the scope of NATOPS. There's also captive carriage and seperation test for weapons. There's also every flavor of system integration testing and software regression testing. Some of the flying can be really unique, and some can be similar to an FCF. It all depends on what's going on at the time.
-Most RW guys go to HX-21 out of Pax. I didn't know that VX-9 was an option for USMC TPS grads, but I guess that's an option based on Rotorhead's gouge. VX-9 does OT vice DT.
-Being a TPS alum opens up doors in to the test/acquisition world, or at least it does for USN types, not sure how it works for USMC. For USN, there's plenty of opportunities to return to the test community in some form either at HX, VX, TPS instructor, or one of the PMAs.
-All the alums I've ever met have spoken highly of their time in the test community and loved the opportunity to get to do some unique flying.