Haven't used MegaSquirt, but did replace a Holley with donor TBI parts from a 1993 5.7l onto my SBC. Way worth it and pretty easy..
I used the Holley projection throttle body system before. Works much better than a carb. Don't know anyone that used the mega squirt system.
Once you have had fuel injection on a hot rod it is pretty hard to go back to a carb. I presently have a "Fast" fuel injection system on my 34 Ford which sports a 383 stroker. I really like it. Mega Squirt, as far as I know supplies parts and know how so you can build your own system and program it. Not quite as easy as the packaged "bolt on systems".