I've been hacking about with my ReadyNas Duo. It's got a little Sparc based CPU (an Infrant Technologies neon), which for the most part sits pretty idle on my machine (despite some hefty NFS access from time to time).
In addition to this, I have an HP Compaq DC7100 desktop machine (which runs Coofer Cat). I do a bit of database work, and thought maybe I could stick MySQL onto the ReadyNas, and perhaps make things work a little quicker. There are a few details about getting MySQL to work on a ReadyNAS, although it's not all straight forward because the ReadyNas is such a cut-down system (so doesn't have some of the tools you might want).
Anyway, now I've tried it, I have to say it's been a bit of a failure. The CPU in the ReadyNas is very small (Linux reports 186.36 BogoMips, which isn't bad, but nothing compared to the 2x5590 in the DC7100). As a result, whilst the ReadyNas can be used for many things, it's not really much good for anything other than streaming data from disk to the network (which is, after all, what it's designed for). I was wondering if I could install some crypto tools and encrypt disk data, but I'm wondering if even that might be too much for the poor little CPU.
Despite all this, I still love my ReadyNas - I'd very much recommend one to anyone who needs a bit of disk storage for photos, music or whatever. If you're a bit more intrepid, like me, then it's still got plenty of scope for a bit of 'customisation'.