I don't have that particular one for the Thunderbolt, so I don't have anything to say for that one.
However, I did happen to buy their
Mega-Grip model, which functions as a universal mount for all kinds of phones due to its flexible and adaptable nature. My thoughts on it?
My Thunderbolt is currently sporting an extended battery along with the Seidio CONVERT CASE for Extended Batteries. If you have seen any of those in live-action or any real pictures of it, you will know that it adds on a lot of bulk, which was something that I was concerned about while waiting for the Mega-Grip to come into the mail.
The Arkon Mega-Grip holds onto it very well, though I have not thoroughly tested it yet (like sharp turns, sudden stops/acceleration and whatnot). As long as the grip stayed in place and the Thunderbolt was placed securely, I had no issues at all, even with the thick, bulky case on it. The bulkiness of the case does cause the Thunderbolt to stick out beyond the legs slightly (I may modify it to have it extend further), but it didn't have an issue since the majority of the weight was still sitting on the legs. If you're a regular OEM battery user, I imagine this will be of no issue for you. I bought this on
Amazon for about $15 free shipping due to Prime (yay for being a college student!). Also, there is access to the USB port.
I hope that helps. Cheers!