The latest version of Opera Mini (version 2.04x) is quite stable on my Treo650. The earlier versions saw some resets when performing things like saving bookmarks.
It seems to handle the Treo's navigation Dpads better too. It's still a great performer and renders the pages faster than Blazer. I am amazed how tiny it is in size. I only miss the ability to Copy and Paste - I had to retype out the URLs when transferring these over from Blazer.
Here's a screenshot of the FIFA WC page as viewed on Opera Mini:
