It was my "second default" browser on WinMo, with Opera Mobile being default (because it could open files on my MicroSD card too). I use Opera Mini a lot on Android, too, even though the built-in browser is quite good.
It is wickedly fast as it is really just a front-end to a proxy server. All the rendering etc, is done on their servers, and a highly optimized result is sent to your device. Granted you lose some content and formatting, but...
I don't want to stray too far off-topic but I found this curious. I'm researching new carriers for when I leave Tmo and came across Simple Mobile. I mention this carrier because they support unlocked GSM phones and among them the Blackberries. Since they don't have Blackberry service yet, they give detailed instructions on how to load the phones with Opera Mini for internet access. Bit of a curious workaround but since I have an old 8800, I may give it a try.
For WinMo, yes. For Palm... not worth the effort. The JRE on Palm and the memory management on Palm just aren't able to handle this well. An exercise in frustration.
Tho, using the JRE on WM, you lose the ability to set as default browser.
Well, you actually can set Opera Mini for Java as the default browser on Windows Mobile, though it takes some doing. I doubt anyone has bothered since they came out with the native WM version, as it's not worth the effort.
With Blackberries, there's no native version available, but the Java runtime is more tightly integrated with the OS providing a better experience than running Java on WM.
Short answer: You need to use a third-party program called MortScript. Since you can't pass an address through the Java layer, what this does is when you click a link, say in an email, it lets the specific script intercept the URL call, launch Opera Mini inside the JRE, then actually paste in the requested address, like a macro. It worked relatively well, for being as much a kludge as it was.