I have a 16gb iPhone with AT&T.  With AT&T you have to sign up with the data plan with the iPhone and it's $39.00/mo and it's unlimited data.  I got the iPhone referbished for $100 under the 2 year plan.  My Razor broke and I had to get another phone so $100 to have an iPhone was great for me.

Screen is a bit small for the forums, but I have used it with lots of zooming in and out and swipping back and forth and up and down to read a long post.

regular YouTube streaming vids are no problem but tutorial videos are pretty much audio only.  I have a Gnomon vid tutorial and threw it on the iPhone and you basically just see the screen but can't really make out what's being done on it - audio is fine.

but watching cats paly piano on YouTube vids, well there's no problem there.