I commute every day via train and I have several devices to keep me entertained: Kindle (3G but I don’t really use it), iPhone (3G), and laptop (offline). Occasionally I need connectivity for something that my phone isn’t quite good enough for. I don’t want to pay an extra $20/month for tethering when I only would use it a few times a month. One day, I realize I could go into att.com and add a tethering plan, get on the train, use tethering, and then remove the plan when I got home. It was great; except that it required planning ahead, and navigating AT&T’s website. You could do it from your phone if you didn’t plan ahead, but it’s rather difficult to use AT&T website from a regular browser, much less a phone.
Then my girlfriend’s brother-in-law was talking about how the AT&T app for iPhone lets him pay his bills from his phone. I immediately downloaded the app, and was pleased to find out it also lets you upgrade/downgrade your data plan in about 5 clicks (or should I say touches?). Way faster than the website, and usable while I’m already on the train. Win!
AT&T prorates the charges, and since it’s $20/month extra; the extra charge I get changed about $20 divided by 30 days, divided by 1-2 hours out of a 24 hour day. My bill usually shows about a $4 extra charge, so I guess they probably round up a little on my hours with the feature. I think it’s worth it.