I bought my iPhone 3G on July 11. Since then this has been my experience:

• 400 mb data transfer / month is more than sufficient for my needs. I'm using in the range of 30 mb per week. There are a couple of reasons for the low usage, prime among them is that I use the internet mostly at home where I have wireless access. An unlimited data plan would not be of use to me.
  UPDATE
  It looks like I'm not alone. MacNN reports that 91% of Roger's users use less than 200mb / month [1]
  
• 16MB storage is the right amount for me. I currently have loaded 925 songs, 983 photos, 8 videos and 27 (all free) applications leaving 7.17 GB available

• Last month I used 120 minutes talk time of my 150; this month I'm at 27 minutes
• My only complaint relates to battery life, but I have a charger at work, home and in the car so I usually don't run into any problems.