I just got my BB 7520 (Nextel) last week - longtime Nextel user here as well.
The unlimited data package means you can surf the web, send and receive email/IM's, etc. for unlimited bandwidth. (To give an idea of why you might want this - one day you get bombarded with email attachments of pictures or something and view them and they suck up a bunch of your data bandwidth... next thing you know, you're paying out the nose for the overage charges). The unlimited plan was only $10/month more for me than the limited plan - well worth the peace of mind.
For web browsing, I found that the "Internet Browser" app that the Blackberry came with was a little flaky and didn't support graphics or advanced pages (JavaScript, cookies, etc) very well. So, I bought the WebViewer HTML web browser. You can get it through
http://www.handango.com. It's a 3rd party app but web pages look generally like they do when viewed from your computer (obviously adjusted for screen size, and most pages require quite a bit of scrolling down). But, that's the nature of web browsing on a PDA, and I really like the WebViewer application. Definitely worth the purchase compared to the built-in browser.
For instant messaging, I bought a 3rd party app called IM+ which works with AOL, Yahoo, MSN, Jabber, and ICQ. You don't need a BES enterprise server to run it. That too has been a worthy purchase - my Blackberry didn't come with any IM software.
A free application I found that I really like is Berry411 - " Berry 411 installs an icon on your start page that gives you quick access to yellow pages, white pages, Google, movie times, weather, encyclopedia, and Froogle results." Find it at
http://www.thebogles.com/Berry411.htm.
I picked up a couple games over at handango.com too.
I ordered the Mapquest FindMe application/service from Nextel on the phone the other day, and they said it would be available to me on my BB within 48 hours. I'm still waiting for it... but it looked much more useful to me than TeleNav for GPS (unless you want the talking directions - but I'm fine following a map), and it's less expensive per month than TeleNav ($4.99/mo I think it was). Of course, you could use them both if you wanted. Here's the link to the Mapquest app -
http://findme.mapquest.com/
Enjoy your new Blackberry! I love mine so far.