Depends on what you use it for. I use WiFi at home, only. During the school day, even though there is WiFi, I tend to just not go on the internet that much. I have my laptop, so I'm good in that regard. However, whenever I unlock my phone, my weather widget updates the temperature, I occasionally check my Twitter/Facebook/Email, but that's it. Any major downloads wait until I'm on WiFi, as that is a lot faster anyways.
I've yet to go over 50 MB. Like I said, I use it lightly when I'm not on WiFi, but even if I didn't, I still don't use the phone that much during the day anyways, as I like to maintain battery.
My plan: Unlimited. Why? I switched over from a messaging phone that had the bundled unlimited data+text for $30/month. ATT has yet to catch that I'm using a smartphone (again, I'm being wary) so I'm just riding it out as long as I can.