I know of several SME installations that have been running for a number of years, it's simple enough to get your head around if you have some basic networking knowledge, and best of all, if you do it yourself you can support it yourself.
SBS is particularly painful to support, ESPECIALLY if someone else set it up. ISA is rubbish, not to mention Exchange. And cost most definately IS a big deal - if it isn't, what are you doing setting up a server for your office, go play golf in Barbados you overachiever you. I'd rather spend that cash on decent desktop OS's for your users, or to upgrade hardware.
EDIT: A note on redundancy. SME supports software RAID up to RAID5, as well as a range of hardware RAID controllers. Pitbull, that means that you have two or more drives in your server, containing all your email and data etc, if one breaks, you simply swap it out for a new one. It really is that simple.
'Nother EDIT: Did I mention remote access and VPN support?