I am going to install this in my E30.
I replaces your entire ECU, and can control ignition timing, fuel delevery etc... Supports WB and regular O2 sensors etc. You can adjust everything and never have to think about "chipping" your car again
Check out a forum specific to your car to find out what parts you will have to swap out. On my BMW for example, I have to swap out the TPS and install a MAF.
Also, download the software and have a look, you don't need VEMS og Megasquirt connected to do that. I really shows how much you can actually do with it.
Also great readouts to display on the carputer (RPM, throttle position, temps etc...)
BTW: The VEMS documentation kind of sucks... Have a look at the Megasquirt documentation, VEMS is based on that but does a whole lot more.