I made the switch too. I never came close to using 2GB when I had unlimited and now I just turn on/off my tethering plan when I need it. The fees are prorated so it isn't costing me the full $20 extra/month (for example I turned it on mid-month this month and I'll pay roughly $10 extra and have roughly 3GB between devices). Months I don't need it I'm actually saving $5 compared to what I was paying with unlimited.
Are you certain that the pro-rated aspect doesn't just apply to the 1st month?
Also, when you switch *to* the tethering plan, how quickly does it get "activated"? If I'm on the 2GB plan and out and about, and I decide I want to tether, can I switch to that plan and be tethering within a few minutes?
Right now my wife and I are both on unlimited, but I just bought my wife an iPad 2 (I think I'm going to wait a while before I get one). She's indicated that she'd like to be able to use the iPad at work and it's a big company with draconian security policies, so she can't connect it to their WiFi. She's a minimal data user on her iPhone 3GS (last time I checked she was at 200MB or less), so I'm thinking of staying with unlimited on my phone and switching her to the tethering plan, or switching her to a cheaper plan and toggling her plan to/from tethering as needed.
With the 3GS she'll have to use Bluetooth to tether, so we'll have to see how that goes. Worst case, we'll upgrade her to an iPhone 4. Actually, I'll probably upgrade myself to the next iPhone in the summer and give her my iPhone 4.