The classic problem and surprise with ZFS's version of RAID-5+ (raidz1, raidz2, and so on) is that you get much less read IO from your pool than most people expect. Rather than N disks worth of read IOPs you get one disk's worth for small random reads (more or less). To date this has mostly made raidz unsuitable for general use; you need to be doing relatively little random read IO or have rather low performance requirements to avoid being disappointed.

(Sequential read IO is less affected. Although I haven't tested or measured it, I believe that ZFS raidz will saturate your available disk bandwidth for predictable read patterns.)

Or rather this has made raidz unsuitable because hard drives have such low IOPs rates (generally assumed to be around 100 a second) so having only one disk's worth is terrible. But SSDs have drastically higher IOPs for reads; one SSD's worth of reads a second is still generally an impressively high number. While a raidz pool of SSDs will not have as high an IOPs rate as a bunch of mirrored SSDs, you'll get a lot more storage for your money. And a single SSD's worth of IOPs may well be enough to saturate other parts of your system (or at least more than satisfy their performance needs).

(There are other tradeoffs, of course. A raidzN will protect you from any arbitrary N disks dying, unlike mirrors, but can't protect you from a whole controller falling over the way a distributed set of mirrors can.)

This didn't even occur to me until today because I've been conditioned to shy away from raidz; I 'knew' that it performed terribly for random reads and hadn't thought through the special implications of changing raidz from HDs to SSDs. I don't think this will change our general plans (we value immunity from a single iSCSI backend failing) but it's certainly something I'm going to keep in mind in case.