Tried Opera Mini and promptly deleted it. Bolt is ok, but I still like the BB browser. And everyone has a different opinion, some hate the native browser and swear by one of the others. You just have to try them and see what you like.
The BB browser if fine, if you plan on visiting the mobile version of a website. As soon as it tries to render full HTML, however, consider taking a washroom break, because it will be a while.
Opera Mini has IHMO the best rendering quality of any browsers that are available for BB, and it runs circles around the BB browser in terms of speed when visiting any HTML site.
Bolt is the fastest one out there, however, the cursor responsivity isn't the best, and the render quality of the text, leaves something to be desired.
As stated before, your best bet is to try them and see which works best for you. What works good for some won't necessarily work good for you. I found Bolt to be absolutely horrible while others swear by it, so there's no telling how it will work out for you until you try it yourself.
My fav is Bolt. Its way faster than any other browser. Sometimes it won't let you download things you have to switch to your bb browser. My only problem with Bolt.