ok, I have looked into linux based solutions for both my employer and our clients, however I haven't yet found a better solution then SBS.
Realistically if you want something to work and work with ease, SBS fits the bill
If your doing the linux route, then from a support point of view, especially if you left, you would want Red Hat installed or maybe Cent OS, now Zimbra looks great, however to get the functionality of Exchange server, you will need to purchase their professional edition, which is a per user per year subscription, so in the long run you are paying more.
As SBS comes with 5 CALS, a 10 user pack is about £285 + VAT which isnt a massive amount and then it's just down to server hardware and your time to add the PCs to the domain, if you already have office, then you have seemless integration with Exchange and happy users and not to mention a gui based backup system that just about anyone can use.
My 2p