Opera Mini seems to be an application that was built just because they could do it. It's really only useful for a small number of people -- those that only have access to really bad Edge data. And it needs to be really bad to make Opera Mini useful. The goal of Opera Mini is to speed up the rendering of web pages by doing all of the hard work on the server and then sending down low bandwidth images to the client on your iPhone. And, in theory it works, but in practice, not so much.
The plus is that Opera Mini is free to download, and does have some interesting features. The 3x3 grid of quick launch web sites called Speed Dial is great. The zooming and scrolling is wickedly fast. But overall, if a page doesn't render properly, how useful is it?