I have a Mass-Flo on 418 that finally dynoed at 580hp with a very big cam.
It has been flawless, starts immediately even after sitting for weeks, never have to touch the throttle to start, self tuned when we changed the cam, and the thing gets 17+ mpg with mixed blast around and freeway driving. It looks like a jewelery like big carb under the hood.
Once it's in place, you just go. Any mod you do to the motor will either cause more or less air to go through the intake, and the system adjusts to the air flow. Very simple and effective. I went from 535hp to 580 with just a cam change and it adjusted to the change immediately, we didn't do anything but start it, warm it up, do the pull and read the numbers and a/f number, which was the exact same before and after the cam change.
One downside for some is the current system is 1000 cfm and it look a bit restrictive at first glace where the mass flow meter sits above the intake.....but it still lets my little 418 put out some big numbers and has street manors that are great.
The only other downside I have found is the issue of a big overlap cam and lots of idling, and a less then perfect pcv setup. If you suck an excessive amount of oil
into the intake and the lumpy cam spits some of it back up the intake and it coats the sensor wires on the mass flow, the information the computer reads will slowly get bad. The solution is to use a mass flow spray cleaner and just squirt the wires clean.....and fix the pcv puke problem so you don't have to do it again, which I did.
Personally I would avoid anything that required computer tuning (that statement will probably get a hot response). My reason is really pretty simple. This system can make big horsepower, get you good gas mileage and you don't have to mess with it at all, ever.
I wouldn't go back to carbs, choking on the fumes at red lights and 8 mpg for anything. My only experience is with Mass-Flo and their guys which ihas been exceptional. So others that use the same technology might be just as good. This has worked great for me. Turn the key and go, every time.