Have loved it, but upgrading to ATS
I've been using a pirana to rappel with since I started canyoneering about 2 years ago. I like it more than an ATC (which I occasionally use) because it's so much easier to add friction on rappel (just throw a wrap around the horns) and you can get a lot more friction with the pirana than with an ATC (which comes in handy for big drops).
It's a great product, but I see 2 downsides compared to an ATC. 1, it's a lot heavier, and 2, I find it harder to choke up with, (and maybe 3, you can't belay with it, but I only use it canyoneering, so that's not really an issue).
All that being said, I'm upgrading to the Sterling ATS mainly because it has horns on both sides of the device, so you never need to worry about having the brake line on the correct side to add friction mid rappel (the ATS can also serve as a belay device, which is nice too).