I think I've had all of two mid- to upper-grade PoE switches fail on me, out of several dozen in use - one D-Link, one Cisco, both older units. The Cisco would sometimes lose connectivity on several ports (up to half of them) but get it back with a power cycle. The D-Link didn't fail in operation, but if it was power cycled, it would sometimes fail to start up properly.
I have a wide range of PoE switches in service right now, from 4+4 nameless unmanaged desktop switches to 48-port Cisco SGE models, all chunking away happily. There are even a bunch of old Linksys SFE-1000Ps (the first managed switches we used) still in service after 8+ years.
There are a lot of suggestions here of redundancies and backups and splitting up networks and whatnot, but basic risk assessment from MY experience is that switch failure is really not that much of a concern. We have gone with two 24-port models vs. a single 48-port on a couple sites for exactly this reason (so we don't lose ALL cameras if a switch fails) but five years on for this site, there hasn't been a single issue (the site also has another 24-port and an 8-port in remote locations).