I use OM 3.1 for the websites I view the most... mostly blogs like wikipedia, engadget, joystiq, thesuperficial, etc. It is incredibly annoying that the program switches to another screen every text is entered on a website. Having the bookmarks on the home page is great, and having a quick-search for wikipedia is also awesome. But do they really need a 'yahoo' search bar (google?????)? I love the cache too, for I'm constantly going back and forth through pages.
OM 4.0b is very pretty and smooth, but sometimes has trouble loading enitre websites or images properly. If they tweak this for the final version, it will most likely become my favorite browser. The browser keeps websites looking the way they would on a computer screen, and you quickly zoom in on the parts you want to read up close. The program tries to be "smart" too, and automatically centers in on text, then adjusts the text so that it fits across your screen (and makes it double-spaced... for easier reading?). Right now I use it to check pages that are made awkward by resizing. It loads blackberryforums.com, imdb.com, and myspace (facebook too) perfectly. Many people here will not care about this, but just having the 4.0b gives you that "my phone can do everything the iphone does and more" street cred. Sadly, they took off the wikipedia search bar on the main menu :( Also, though I don't know the technical reasons for this, but I've heard that it is not wise to use this browser for any websites that ask for your private information.
Nobody mentioned this, but a large annoyance with the BB browser is that it automatically loads RSS feeds on webpages that have them. Most of the time it wont let you switch back to the actual website. This sometimes cripples key links that you'd want to use, such as comments or pictures. The Opera browsers both attempt to load the original websites. If an RSS feed is available, the link for it is usually at the top of the screen. The only other time I've ever used the BB browser is to see if I can view a higher resolution image on websites.
It won't hurt your phone to have all 3 browsers. I keep them all right next to one another.