When GSM phones are in a poor signal area they up their transmit power in an attempt to maintain a connection. It's an *enormous* difference (2W compared to 20mW at the extremes - that's 100:1) compared to being in a good signal area. The protocols involved means these are not figures for constant drain on the battery, but the point is that you are likely to see wild variations in battery life even using the BB the same way, depending on where you're located.
For example, I had poor signal at home. Any/all GSM phones would hardly last a day (even making NO calls!). Then they fixed the network and I get a strong signal. The same phones will now last days
So maybe one day you get caught in a traffic jam for an extra 30min in a poor signal area. The next day you don't. Add to that the effect of making calls, using the backlight, watching videos, use of sleep mode, bluetooth, WiFi, etc., and you can see why it's very hard to make comparisons between users. For me WiFi and Bluetooth use together maybe affect my end of day battery life by 10-20%. But if WiFi is saving high power GSM transmissions, overall battery life will improve by switching WiFi on!