Depends. When I treid Bolt, it was great on display but 2nd best on reading, IMO. I access the web on my BB mostly for reading (CrackBerry, etc.) and Opera Mini juggles everything into a very readable form (no side-to-scrolling). Bolt seemed much slower in page loading in my experience.
I have both Bolt and Opera Mini installed. I use Opera Mini the most but sometimes I will use Bolt instead.
If I want to browse the web at lighting speeds I will use Opera. But if you have a heavy image site you may not be able to view all the images properly....
If I want to go to a site with tons of images and want to see each image then Ill use the Bolt browser. The Bolt browser gives you a near desktop experience but it loads alot slower than opera mini.
The only time I use the bb browser is when I have to download an app OTA.