I'm wondering if anyone in this thread has actually gone into their AT&T account and looked closely at their data use. I know you can do it with a built-in contact widget (you call for "data use" and get a notification), but that's pretty generic, just gives you a total up to that point in your billing cycle.
To really get a handle on data use, check out your AT&T account online (Web - if you haven't established an AT&T Web account, by all means do it!!!!). It's actually got a lot of telling info on it (even for the controlling hordes at AT%T!). Looking at your "data useage" link in "My Account" on their site will actually give you a time and particular data load. I found in looking at that stuff that every 15 mins. I'd get 3KB worth of syncing of God Knows What. Now 3KB isn't a whole lot, but figure every 15 mins. makes 12 KB/hr., or (multiplying), roughly 288KB per day, or 8.5MB a month just for being on. Not a huge portion of the allotted lesser plan (200MB), but it adds up.
My data hogging is mostly for Web surfing, in concentrated sessions, i.e. once a day for at a specifc hour, etc. In those sessons I've managed to burn up about 30MB in a week or so. So I'm right on the cusp of staying on the cheaper plan. I think of it as disciplining myself to surf without wasting time and effort. As it stands, I make it "under" the maximum by 2-3 MB per month.
Anyway the point here to REALLY find out, because you're not going to just from others, who use it in a variety of ways:
1) Establish an ONLINE (Web) AT&T account if you haven't already
2) Log into your account and click "Usage and Recent Activity"
3) There should be a list of all phones (if > 1 in your plan). Scroll to "Data Usage" (or similar) section.
4) Under the proper phone (again if > 1), click "View Details".
5) You'll get a page with a bunch of different info, including EACH data use instance (much like a phone bill list of calls).
6) Note the dates, times, and amount of data. You can track when you glutted out vs. other "automatic" uses.
It might be a good idea for one sitting to just do a few brief tasks, like surf through few pages, maybe look at web e-mail (if you have it), etc. and NOTE IT in writing, the time, page pulled, etc. (your computer probably has a task logger but that's getting kind of fancy, but if you know how to use it...). Anyway just do a few maybe 2-3 minute things, maybe 4-5, then note the amount of data pulled when it shows up on the AT&T usage page 1-2 days later.
It's not really tedious if you just do it in 1-2 short, easily identifable "Web" sessions. Then you can see if you're worred about keeping usage trim where you gorged too much. And the AT&T page shows other data usage (Texts, Mobile share, video share, etc.) so you can look at those and try similar "experiments" to track your data as well.
Note that there is a "Data Usage Calculator" AT&T provides on the site but I find it kind of generic and not really too accurate (it's based upon how you answer questions which can vary broadly). I tried it and found I'd like to use data like I answered but I actually use data differently than I answered, which I only found by doing the "data trace" I mentioned above here.
Just take a few minutes and try jotting down a few uses then checking in your online account page(s)... it's well, well worth the $$$ to give it a dry run. Of course if you don't want to mind your P's and Q's (literally!) then get the bigger plan. I want value so I'm trying to adjust my phone data habits to be a little more efficient. So far so good.
Oh since I also discovered that my phone would still chirp out little 3KB data blurbs every 15 mins. even if Syncing is "OFF", I found a way (also mentioned in this forum) to juse JuiceDefender (even the free one) to FORCE my phone to NOT send or receive any data (not including text and voice phone) AT ALL until I tell it to. And apparently this little tip has worked great! I looked at my online log since instituting this fix and there's no data at all recorded as being sent(!!!!). So now I don't have to turn off the phone to have it stop leaking, and avoid irritating restarts (reboots).
I can provide a link to how this is done (again it's in this site) if interested.
Looking at my online individual data use instances has REALLY helped me understand how I use it, and what uses a lot of it. Again, don't download ANYTHING on 3G, use USB or WiFi if you are able. Most Web surfing for about an hour will burn several MB's, so just be aware of that. I'm trying to give the 200MB limit a go, and so far I can. The $10 per month difference isn't a whole lot but it can add up. I mostly don't want to pay for what I don't use. If I start feeling like I'm crimping too much, then I'll go to the larger plan. So far it's not necessary.