As a working father of two under age 4 I use my iPhone much more than my MacBook. My usage fluctuates from 1.5GB to over 3GB and I use wifi at home. Thank goodness for my unlimited plan.
Yeah, unlimited is not just useful, it's actually liberating - you can do absolutely anything with no consideration for how much data you're going to use.
Well I'm with Three here in the UK, it's truly unlimited. I get pretty good HSPA+ speeds, and one month I used about 14gb in a month as my ISP at home was down for a bit.
Enough to get throttled every billing cycle by AT&T. Once throttled my usage steeps down and hardly anymore mobile data is consumed because the speeds are just unbearable.
I use about 400-500 mb, just over the 300 mb data plan from AT&T. Because of that, i'm stuck paying extra money (albeit only 5 bucks...) for my grandfathered 2gb data plan.
I typically use around 1GB a month, but do reach up to 2GB occasionally. I am probably going to see that usage go up with my Nexus 7. I downgraded from iOS 6 to iOS 5.1.1, just so that I could tether for that... lol.