I have a MegaSquirt system installed on 2 totally different cars, one turbocharged suzuki vitara and one Jeep with a N/A 400ci small block chevy.
To begin with the kits are cheap, parts cost for a board runs about $130 USD, $50 for a partial kit from the 'project daddies' and the rest for other parts from a different source. There are also some other options available, stimulator for bench testing the ECU, relay board for easier wiring and MegaView for a neat looking vacuum fluorescent or LCD display, more of a gadget than a tool really, but we all like gadgets don't we...
If you have experience soldering and such, you'll have no problems assembling the units, but there are people who sell preassembled units for the soldering impaired.
Tuning is done with a PC, using a serial port and various flavours of tuning software that run on all Windows versions and one program being developed for unix/macosx.
All the units come with a 250kpa MAP sensor (2.5 bar) that senses boost up to just over 20psi, a MAF is not used.
The user base is quite big, and the system has been installed on a wide variety of vehicles, and people are quite willing to give their help on the mailing lists.
Originally the system was designed as an easy way to convert a carburated car to fuel injection, hence it doesn't control spark timing but as the user base grew, quite a few code variations have been produced with lots of different features.
There's dual table code for running 2 sets of injectors mapped individually, that code also features rev limiters, launch control, PWM idle control, nitrous safety control (rpm window switch, temperature switch and throttle switch all in one), progressive shiftlight and experimental boost control.
MegaSpark uses standard megasquirt hardware to control spark with a distributor, but no fuel.
MegaSquirt'N'Spark controls fuel and spark with a distributor, plus it has 2 outputs that can be triggered by a selection of variables (rpm, coolant/intake temperature, throttle, MAP and some others). It also has a soft/hard rev limiter retarding spark and dropping fuel.
MegaSquirt'N'EDIS is derived from MegaSquirt'N'Spark, controls fuel and spark with a Ford EDIS wasted spark system.
Then there are some other variations floating around.
The fuel 3d map is 64 sites (8x8) with user configurable scale and linear interpolating between sites.
Fuelling is corrected by intake air temperature, plus warmup enrichments when the coolant is cold, all user configurable.
The default is to use GM temperature sensors but the software/hardware can be adapted to use other sensors.
Tuning is fairly easy and you can take datalogs with the laptop to diagnose.
I currently run my vitara with dual table code controlling fuel with 8 injectors, and rely on the stock ECU for spark. It works fine except the stock MAF is quite restrictive with 10psi boost and I plan to switch over to MegaSquirt'N'Spark soon and just run 4 large injectors so I can just completely remove the stock ECU. I've been running with this system since June last summer and I'm quite happy with it.
The Jeepster has a 400ci small block chevy motor from a 70's Chevy Nova, the motor is all stock except the high compression pistons (9.8:1), slight overbore and an Edelbrock Torker II intake manifold modified for fuel injection, using a gutted carb for a throttle body. Running MegaSquirt'N'Spark only controlling fuel so far, with a recurved HEI ignition for spark until we get another HEI to convert for using with squirtnspark. The motor pulls quite well with this setup, throttle response is good and economy looks promising.
See
http://www.bgsoflex.com/megasquirt.htmlhttp://www.megasquirt.info