I use Opera Mini on occasion on my Nokia and so far have had little issue with it. The main reason I originally put it on was for its very low bandwidth usage, as it uses a proxy which compresses the content before sending it to the phone. There is very little 3G coverage in my area, especially out in the country, so the Opera browser loads pages a lot faster than the phone’s standard browser and I’m sure it would be quicker than Safari also in areas without 3G coverage. Here in Ireland, O2 has a 50MB/day max daily usage policy on its prepay network, so this is another reason I avoid the phone’s own browser.
At least with non “i” phones, there’s no waiting for Apple’s approval.