I've been using Opera Mini 4 on my Treo 650 for the past 3 weeks, and I can only describe the experience as "wow!" It took a day or two to get used to the quirky user interface, but since then I've been completely blown away by the performance of this little web browser.
With the notable exception of Flash-based web sites, I have yet to find a site that Opera Mini can't display. Considering that Opera Mini 4 is a Java app, running on the ancient single-threaded PalmOS platform, over the slow AT&T EDGE data network, the usability of this browser is astounding.
If you haven't tried Opera Mini 4, you haven't experienced the mobile web. Not bad for free software...
After reading about your success, I tried again over the break to get Opera Mini working on my 650 and it worked great! Seems like a little voodoo to get everything installed in the right order, but it worked.
Today I saw this post about Palm essentially dropping support for the JVM:
http://blog.palm.com/palm/2008/01/last-chance-to.html
One more nail in the coffin? Opera was the only decent app I knew of that even used this JVM. I believe the WinMo devices have JVM built-in.
Yeah, I'm completely addicted to Opera Mini now. You can see my comment about the JVM on the Palm blog 
Two things I've notice recently:
debug: in the URL entry box.
