I beg you not to do a FW dip on that rock! They go to a lot of trouble to get the rock to you with as much life on it as possible, and you don't want to kill off the sponges, corals and tunicates and all with a dip. Plus, the mantis may just decide to stay put.
I have done their package twice. The diversity on the rock is fantastic. Sponges, tunicates, macroalgae, corals, and so on. It is truly beautiful and interesting. The service is very good, and the cost is pretty reasonable. There will be die off over the first few months, but I still have porcelain crabs, bruttle stars, worms, sponges, corals, and macroalgae that are growing and healthy from my first batch 3 years ago.
There is a down side to diversity, which you have to be prepared for. Stephen already mentioned one problem, mantis shrimp. I got none in the first package I bought, and 6 in the second a few years later. They were extracted relatively easily because they make a burrow in a rock, and you can extract them once you fnd their hiding place. The upside is that they are very popular and can be traded if you get them out alive. Hairy crabs are also a problem. Once they get to a certain size, they can be very destructive.
Another somewhat odd problem is that there are so many clams, corals and sponges on the rock that some may have to be removed to add your own corals. The last batch I had shipped was covered with tube coral, and I may just frag some of it and trade it to make a little room.
There's a little less risk of baddies if you buy Pacific rock, in which a lot of the stuff has died in shipping. On the other hand, if you want a great experience discovering new species, the TBS rock can't be beat.