There are certain mobile applications that I use on a regular basis regardless of my current device platform. It hasn't been difficult at all to find apps that perform the same function on my new iPhone as I had on my previous Sprint BlackBerry 8830, and in most cases the iPhone applications are more elegant both visually and functionally.
Mail
Hard to beat BlackBerry as an e-mail machine, but iPhone is significantly better than I anticipated. It's a perfect clone of Apple Mail on the desktop, and my concerns about typing on the iPhone touch screen were unfounded.
Calendar
The BlackBerry calendar is about as minimalist as you can get. iPhone's calendar looks exactly like iCal on my MacBook Pro and syncs very well with the desktop. I tried MobileMe but had some issues, and since it doesn't currently sync shared calendars I'll continue with the local sync until Apple fixes it.
To Dos
I was using Rex Wireless' ToDoMatrix on BlackBerry, which is super functional. No built-in ToDo application on iPhone surprised a lot of people, so I've been using Things. Works very well.
Notes
Had been using Rex Wireless' other app, IdeaMatrix, for notes on BlackBerry. iPhone has an ok notepad built in which doesn't sync to anything. Evernote works pretty well.
RSS
I was using Google Reader for RSS feeds on BlackBerry, NetNewsWire works great on iPhone and also syncs with my desktop.
GMail
No GMail app for iPhone yet, but Google has an iPhone-optimized page that works pretty well. I preferred Google's BlackBerry app.
Instant messaging
Can't wait for Beejive's iPhone IM app. Their JiveTalk is the best BlackBerry IM app available. AOL IM is out, but without background notifications any IM app will be unusable on iPhone.
TypePad
BlackBerry and iPhone app are identical, and neither lets you edit previous posts.
Dictionary
Saving my pennies for Beiks dictionaries for iPhone; they're great on BlackBerry.
Password keeper
BlackBerry's built-in password keeper worked very well for me but didn't sync with anything, so you had to type in everything on the handheld. SplashID iPhone/desktop combo works great.
Music
No comparison - iPhone is a big, beautiful iPod! Pandora and Tuner gives you easy access to an almost unlimited amount of fantastic streaming web music, talk, etc.
Photos
My BlackBerry 8830 didn't have a camera, and I missed the one from my Treo. Quality isn't the best, but it's lots of fun snapping quick pictures of whatever. SmugMug's iPhone app is terrific for effortlessly uploading quick snapshots onto really nice looking web galleries.
Web
Mobile Safari is fantastic, especially over AT&T's 3G (which works very well in Northern VA). And there are hundreds of iPhone-optimized sites left over from the first generation iPhone that provided only web apps.
eBooks
eReader on iPhone is stunning! It looks like a small book, and is a pleasure to read. The eReader app connects right up to your bookshelf to download any of the books you've purchased. Very well done. I never read eBooks on BlackBerry because the screen was too small and memory too tight.
International travel
The Sprint BlackBerry 8830 worked very well during my recent visit to France, and did not cost much since I added the international data plan and kept voice time to a minimum. AT&T has similar international plans and will also work in many countries.
Bottom line, you can't lose with BlackBerry or iPhone. Better sync and far better appearance and available applications give iPhone the edge.