I've got a fairly cheap tariff with O2 - about £34 per month that gives me unlimited usage. In the 2.5 years I've been with them, I've never been charged for any excess. Only time you get stuffed for excessive usage is if you use phone as modem - hence why I got 3G card for tablet.
There are a plethora of sites that are mobile friendly, so I can happily check news, travel, weather and tides - very useful for me. I live in the North East and rarely get a GPRS only connection, so happily browse on 3G.
As with Bracken Dawson, I use it all the time as do many friends and colleagues. I dont use Facebook or Twitter as I don't feel the need to see constant updates from people informing me of the colour of their toast or the shape of a particularly "funny" cloud. Pointless, IMHO.
I use 4 email accounts and can happily get all emails, work and social, pushed to me. As a climber/hiker/sailor, having stuff like Googlemaps with aerial photos/maps as well as real-(ish)-time weather has been invaluable a few times for planning/decision making.
Far from being a Jesus Phone type, I feel this sort of technology is complimentary to my lifestyle and is certainly useful, just like my SwissTool or a Maglite, but not the focus of my life.