I had the chance to mess with a few new to me EFI systems over the last couple of weeks including the BS3 and XFI and had a real eye opener concerning the "homegrown" US-sourced EFI stuff.
The FAST, XFI, and BS3 are good "domestic" boxes-- meaning they're somewhat basic and limited on options but will do a fine job if setup and tuned properly. The BS3 is pretty skimpy when it comes to documention, help, and support provided with it but it is a good box. The XFI is much better from a documentation standpoint and offers a very comparable featureset compared with the BS3.
However, I also had a chance to mess with an AEM and an Autronics box, and man oh man are they leaps and bounds ahead of the typical Gen VII/FAST/XFI/C950/BS3 and the price is very comparable. The AEM and Autronics boxes are much more involved to setup but there are a *TON* of user configurable features in them that the "domestic" EFI boxes don't even think about offering. You can run darn near any sensors/inputs/outputs you like and you can configure them on your own; linear voltage, PWM, etc. They also offer acceleration-rate based traction control, multiple input and output triggering options you can configure yourself (no sending the box back to be changed like a FAST), way more advanced temperature compensation tables (i.e. with the FAST you can only change fuel based on IATs, while on the Autronics and AEM you can not only change fueling but change timing and also drive a configurable output like PWM signal to drive a boost control solenoid to cut down on boost due to excessive IAT's.) They also offer flatfoot shifting via a clutchswitch input, antilag for turbo users, etc.
Absolutely no contest IMO. If I were re-doing my car again I would probably go with the Autronics box as I've heard some minor griping with the on-board injector drivers in the AEM box. My FAST works and drives well but there are several options I wish I had that it does not offer. The Autronics box would do everything I want and then some!
And as Frank said, even the best box will be a dud without a good tuner at the helm. I would base my decision on tuner availability and knowledge above almost all else, but do not limit your options to the "domestic" boxes; you're missing out on a LOT of good stuff if you do.
http://www.aempower.com/product_ems.asp
http://www.autronic.com/
Troy