Never heard of them, but site looks pretty professional. No obvious negative feedback when googled, and whois record shows they've been around since 2001.
Spotted this after a quick search, one of their customers. Maybe worth contacting them to get an impartial opinion, see how their relationship panned out.
Lessons Learned: Stacks and Stacks' Cathy McManus | Practical eCommerce
"Search engine optimization
