Well, my buddy has a 3G iPhone and just got a Bold for work. He can't DL 3rd party apps, so that was one downside for him (work limitation). You wouldn't have that I assume. His only other downsides were, he preferred the iPhone web browser and battery life. However, he has yet to update his phone so forget battery life also.
Basically, just the browser. I have absolutely no issue with the browser, i mean it's a phone... I have only played with the iPhone's browser for a min or two and that was long before I had my Bold. I had a Treo 750 at the time.
I would assume the iPhone might have more apps, but they seem to be rolling out nice stuff for these Bolds.
I currently have the iPhone and made the switch to the Bold (still waiting for it to come in the mail). I really like the iPhone but I think the Bold is more my style. They are both great phones but just comes down to what your tastes are.
I had an iphone 1st gen and it was good but after awhile to me all the iphone becomes is a toy. I love the bold because it actually has alot more functionality when it comes to email and linking it to things like facebook and your accounts. I wasn't even able to do that on my original iphone and once I did end up figuring it out emails took forever to receive. I do miss the bigger screen and better graphics on certain apps but in the end I love the bold. BBM is a big bonus. I never had the pleasure before.
I would have to agree with most. I had both phones at the same time for a while and IMO, the browser is better on the iPhone. The iPhone, if its jailbroken, has a plethora of apps for the taking. Most of them are very functional. After a while it felt more like a toy than a phone and battery life SUCKED. I got more battery life with my blackberry than I did with my iPhone. I ended up taking it back before my 30 days was up and bought a 2g iPod Touch.
He who laughs when something goes wrong has already found someone to blame it on!
Had a bold went to iphone 3G now back to the bold. iphone is great but the onscreen keyboard just isn't for me. considering its the #1 thing i use on my phone is the keyboard (email / text) I also feel the bold is a better music player! I can type the name of a song and boom its right there instead of playing the finger game. Only thing you will miss is iphones web browser. 39800 of the 40000 iphone apps are worthless but thats IMHO. Hope that helps.
edit: I wanted an 8900 as well for better camera and size but i have come to love the bold size (i do have larger hands) mainly, i need 3G (tethering)