I replaced a drive on my Thecus NAS last year. It was simple, but you do have a period of some hours while it's rebuilding the parity info where access is a lot slower.

And I misspoke -- I'm not actually using RAID5, I'm using Synology's "Hybrid RAID" which is much more flexible when you're replacing drives, adding larger drives, etc. I haven't had to replace a drive on this one yet, but when I do, it'll be nice knowing I won't have to use a drive of the same size (or waste the extra space beyond the size of the smallest drive in the array.)

I don't have an opinion on the Squeezebox question -- I still use original Squeezeboxes with the empeg-esque low-res display, and haven't really looked at the rest of the lineup recently.