If you're talking about GSM Blackberrys, I'm not aware of any 3G model.
I have a Verizon EVDO (CDMA 3G) Blackberry, and when it comes to the experience of browsing with my iPhone on EDGE or my Blackberry on 3G, I much much much much prefer my iPhone on EDGE. The overall experience of loading a page may be slower on the iPhone, but the page is rendered in a much more usable way for me.
IMO, even though it's first generation, the iPhone's been out long enough to be proven (one way or the other, depending on your specific needs). Any question you have about its abilities in a business environment has more than likely been blogged about (by tech pros like computerworld, cnet, etc) or forum-posted about on Mac sites like here and on Blackberry forums. I'd be surprised if you don't get a general feel for whether or not it'd work for you just by doing a little googling (or forum posting, like your question here).
Where I work, I can't use the iPhone to replace my BB. My company doesn't allow external IMAP or POP3 access, so the BB is the only way I can get my work email.
I used to use my work BB as my main phone, carrying it everywhere. Now I have it call-forwarded to my iPhone, and I carry my iPhone everywhere. I usually have my BB laying around in case I have to read a work email, but I only find myself using it occasionally now.