I have an AEM EMS, which has allows full control of your engine electronics as well as datalogging. It also has some cool stuff built it like a progressive boost controller, and several outputs which can be used for all sorts of things like controlling multi-stage nitrous kits and such.
Also, it is relatively easy to install vs something like a FAST XFI or Big Stuff 3, but not as easy to use IMHO.
Paul.