I've used both. Of the two, I find Boingos the more adult-friendly. The difficulty with Snappis is that a single Snappi provides minimal support to the front of the diaper. That tiny central claw just isn't enough. As a result, over a relatively short period of wear, your diaper will get loose around the legs. That's not always a catastrophe, but I definitely prefer a more secure-feeling fit. I also don't often mess in my cloth diapers. If I did, I expect Snappis would be even less suitable for the lack of containment. Some people like to use two Snappis, one per side. You can definitely get a snug fit that way, but man... So fiddly!
Boingos aren't perfect either though. Keep in mind that the intended wearer is a baby, and a baby is almost certainly wearing a close-fitting hook-and-loop or snap-on diaper cover, not the baggy pull-on plastic pants so many adults wear. That distinction becomes important especially if you sleep in your cloth diapers. They'll inevitably loosen over time and as you wet, and if your cover isn't holding the Boingos snugly against the diaper, they'll just fall right off. I've had good luck wearing Boingos during the day thanks to gravity and my outer clothing, but over night they'd always fall off.
So yeah. I've tried them both, and I still occasionally play with them, but I always get a better, more secure fit with good ol' diaper pins. And who can resist those cute duckies?