I tried Opera Mini on an older series 40 handset – a Nokia 6100. Apparently you get a different browser for each handset: the application is still called Opera Mini version 1.2, but its much smaller (63KB while the version installed on the 6280 is 100KB) and it has a bit different user interface:
* The settings page is smaller with much less options.
* No smooth scrolling
* The “select” key is ‘*’ instead of the directional selector which is not available on the older handset.
* There is no chrome at all: the API for j2me (MIDP/1.0, I think?) doesn’t let you use one, so you get an “Opera Mini” title at the top and the bottom always have the multi-key assignments.

Its also a bit slower and less responsive then on the 6280, but its still plainly much better then the builtin browser and is good enough. I would use it if I had a 6100.