Prowler dismissed the Fi Q as a forum boner and suggested that members would be able to offer better reviews on other woofers. However, I doubt that you'll be able to find another SQ-based sub that has been tested by more members here. I don't have any personal experience with this driver, but I haven't heard ONE bad review. You really can't go wrong with it IMO. Plus, Fi's customer service is great. I dealt with Scott a bit while trying to pick a sub for my truck. I didn't end up going with Fi, because the Q is a bit too deep for my application, but I would not hesitate to purchase from them in the future.
Incientally, If UPS doesn't get they're shit together soon, I may have a TC9 12 for sale in the near future if you're interested.