Hosted Microsoft platform hands down.

How much is your time worth? You have no dedicated IT staff and should be spending your time working on your company's product.

Yes, open source Exchange alternatives are getting better, and I really like Google Apps. How much time would it take to tweak the integration? How much time to develop workarounds for features that "just work" in Exchange? For a small organization you'll never get that investment back in cost savings.

$60/mo for 10 people is well worth the service you receive. Even the $200/mo for the full Exchange/Office bundle may be worth it.

Exchange is a great candidate for virtualization if you are running a small shop, but then your better option may be one of the various hosted/cloud Exchange offerings. (And you'd better not tell MS about your virtual environment when you call for support)

In a large installation Exchange will use an incredible amount of I/O and cpu. I just don't see any value from running it on super-reliable hardware. You have to plan for Exchange breaking due to other problems.. corrupt databases, broken networks etc. which means a "hot spare" system ready to go.

So if you have Exchange with CCR with a 1-2 minute failover max, why bother with reliable hardware at all?