I'm never anywhere for more than a few moments where there is free WiFi, so I actually don't think I've ever used anything but 3G.
I have to say, though, with the Netflix streaming - BOY AM I GLAD I KEPT UNLIMITED! I've had my 3gs just over a year. The first few months I used about 1GB a month but after that I settled between 400 and 800MB, and even less once I got my new laptop a couple of months ago.
I had been very close to changing plans - in fact, I almost did last month I just forgot to do it. I was going to go to the 2GB/mo plan, which would have saved me five bucks a month and also made me eligible for tethering which I do not need on a regular basis but would have turned on just for a month at a time when I needed it (vacation, etc.).
Wow how things have changed in the last couple of weeks. First, I used 500MB streaming a few movies in the first two days of Netflix being available. I was a little shocked myself. I watched like two movies and a couple of episodes of a TV show. I can use my iPhone at work for music/video, but I've never bothered much with video because it's a pain to transfer over shows for 1-time watching, and how many times do I need to watch Star Trek on digital copy. With Netflix streaming, though, it's amazing - while I'm working away I can watch virtually anything I want (plus it's finally given use to the landscape mode of my iZel stand).
In addition, Virgin Mobile just put up their 3G Unlimited for $40/month. You just buy their little dongle for like $80, and pay-as-you-go for the $40 internet. That's ideal, because I'd probably only need it 2 or 3 months out of the year and wouldn't have to bother changing stuff with AT&T all the time (which always seems to cost more than it should when you change features). So there goes my need for tethering.
I will be very curious to see how much data I use this month because of Netflix. Either way, I feel like I made the right choice staying unlimited.