I came from 2 BlackBerries, too, and thought I'd never get used to the touchscreen typing, either, but now I'm at least 50% faster on my iPhone than I ever was before. I first had a 32GB 3GS and then went to a 32GB iPhone 4 last summer, only because there was no 64GB version available (due to flash memory costs, apparently). I am crossing everything I own that that'll be available this summer!
As for what I love about it, I love that for a while, I had consolidated my mp3 player and phone into 1 device...unfortunately, I've since acquired so much more music and so many more videos that I ended up springing for a 64GB iPod Touch and that's what I use in the car and to connect to the TV to watch videos and movies. I use the iPhone when I'm working out and also as a point & shoot camera when I don't feel like bringing along my actual P&S or my DSLR. It's actually got a quite acceptable little camera packed into it, which has come in handy when I'm out walking around or at the parks, etc.
The variety of apps available speaks for itself. Some of my favorites are as follows:
Hipstamatic ("toy" camera app)
ProCamera (allows more control over exposure, focus, white balance, has a self-timer, etc.)
Osfoora (Twitter client)
Facebook
various Disney-related apps: WDW Maps, WDW Times (electronic version of the paper Times Guide available at the front of every park), eTicket, Disney World Hidden Mickeys, Wait Times, and WDW Dining
Games: reMovem, Toy Story Mania (SO similar to the ride & Wii game it's startling), Bejeweled 2, Stair Dismount

, and Sol Free, a free solitaire game
Photography: Photoshop Mobile, PanoLab, Photobucket, Flickr, Camera+, Color Splash, Pro HDR, Tilt Shift Gen
Other: Southwest Airlines, Live Nation (for concerts), eSeats (for concerts & sports), iConcertCal (pulls artist info from your iTunes library to compile a list of upcoming concerts by your favorite artists in your local market (or any market you choose)--you can buy tickets to shows and get maps to venues from within the app itself, which is really handy), StubHub, Travelocity
And if you should get an iPhone and decide to jailbreak it, there's a whooooole other world of stuff you can do to it.