The difference is that you can know what's wrong and fix it (assuming you're able). Also, over the past 2+ years, my mail server has had less downtime than GMail, and any outages were easy for me to diagnose and fix. I like feeling in control over critical pieces of my business (at least as much as possible - I'm in no position to build my own DC for instance).
I know google hires some people who are better than me, but that doesn't mean the gmail system is better than something I could engineer.... For one thing, google needs to have a much lower cost per user than I do, forcing them to make some choices that I don't have to. I have no problem paying $50 a month for email service, and that level of cost per user would be untenable on a free email service.
(I host most of my email myself, and I'm trading a free co-lo to mark perkel of junkemailfilter.com to do my filtering. between that and my time, I'd guess $50/month)
