It has been over two weeks since I switched our mail to Google Apps, and things have been working pretty well. The only problem that we have encountered, is that more spam has been getting through. Previously, I had SpamAssassin with bayesian filter configured to scan our mail. With enough training, we got it working well.
One thing that I would love to see in Google Apps for your Domain, is the ability to import a SpamAssassin bayesian tokens database. This would allow the spam filter to be tuned for a particular domain, as soon as it is switched to Google Apps.