Back in my Omnisky beta testing days, Avantgo was a definitive no for the test. It caused nothing but problems on all of my other Palm devices that I just opted to carry around the laptop if I wanted internet data.
Course now with the Treo, it's a mute point. And what's amazing to me is that I have a device that's smaller then the Omnisky modem/Palm Vx footprint and connects to live web sites at about 105,000k compared to maybe 19,200k.