Cisco ASA
I haven't used them in a few years, but even back then, ASAs were hopelessly outdated junk. Their UTM functionality was a joke, ASDM was barely functional, and they were slower than the competition. Cisco's new ASA 5500-X lineup seems to have made their performance competitive again, at the cost of even less UTM functionality

If anybody knows what the draw of these devices are, please chime in. I simply can't see any compelling reason to use them at all.
SonicWALL
I haven't really used these devices much, but based on what I've seen, I'm not a huge fan. In what I assume is an attempt at making their firewalls easy to configure, they seem to have created a "black box" that doesn't behave as a network engineer would expect. It didn't help that the documentation was very sparse, not context-sensitive, and even missing in some aspects. In fact, their entire GUI could really use an overhaul.
If I had more time to play around with them and get used to their idiosyncrasies, I might warm up to them more. They ultimately work well enough, but they wouldn't be my first choice.
Fortinet
I've generally been happy with Fortinet. The products are fast, feature-filled, reasonably-priced, and they don't nickel-and-dime you on licensing like some other vendors do.
There are some rough areas. For example, logging and reporting are definite weak points compared to the competition unless you purchase a dedicated monitoring appliance or use their monitoring service.
Overall, though, I recommend them and they're usually my first choice for edge security.