As your organisation grows, at some point you'll have to make the decision on whether or not to manage your email yourself. For us, that was when we were at a staff of three - but then, we're a bunch of nerds and that sort of thing appeals to us. For the rest of the world it's probably when you get to around 20 or 30 staff that the benefits of in-sourcing your email become apparent.
However, unless you happen to live in a data centre with redundant power supplies and staff on duty 24/7 you're going to be moving from a very high level of reliability to a (somewhat) lower level. That's ok though - with a UPS and good backups the benefits far outweigh the drawbacks.
We recently had another ADSL line installed in our office which gave us the opportunity to increase our mail reliability even further. Now, if our main connection with IDNet goes down (although I can't remember when that last happened) mail is delivered to the secondary MX record hosted on our new connection with O2. This server (a woefully overpowered and underused Intel machine) collects email for fluid-it.com until the primary MX comes up again, at which point it dutifully hands it all over.
One caveat with this setup is that spam relays are often configured to send mail to the backup MX records - in the hope that they'll have less strict spam filtering. Fortunately for me, spam filtering with Postfix is as simple as adding blacklist records and installing postgrey for greylisting...both those are for future articles!