I almost bought from them for my recent conversion. I have not used personally used them but I read that TS produced a bad batch of batteries a few years ago and did not warranty them. I think that they have cleaned up their manufacturing processes, but I am not confident of this, nor if they improved the customer relations/warranty policies. Check
http://www.evdl.org/. Also there is a yahoo group specifically for thundersky, they should be able to give you better information. I do not remember the link, but I found it from the evdl.
Good luck.
Nathan.