Personally I hate when businesses spell their names with a Z in place of an S. Such an unprofessional image...
When it comes to domains I would avoid them, because whenever you (or the end user) tells someone your website address, you'll always have to say "oh it's spelt with a Z on the end", which is lame.
I note that Fairfax have done well with Stayz.com.au, but they also have Stays.com.au which redirects, so they're still catching anyone who types in the proper spelling rather than their stupid misspelling. I reckon most Z domains would lose a heap of type-in traffic to their properly spelt competitor's websites.