Does anyone have sub-frame connectors on their car? What benefits have you been able to notice?

Several members on the board have SFCs. Almost all say that they notice the car is tighter with them and some also say they help with interior rattles and creaks. Most importantly though, it strengthens the unibody chassis and reduces the amount of flex the car endures. It's definitely the first mod anyone should do to a thirdgen. There are several good brands of SFCs out there. Just do a search on SFCs and you will find enough to keep you reading for a very long time.