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While the iP4 and beyond are a tad on the uncomfortable side to hold when I'm in bed laying on my back in certain situations, I prefer this design for when I'm making calls, especially in the heat of the summer. With the 3G and 3Gs I found I would hold the phone closer to my palm and if it's really hot outside, sometimes I would come away with a nasty plastic back. With this newer design I generally hold the phone just with the tips of my fingers, leaving a big airflow gap between my palm and the phone.

Sounds gross, I know, and no I'm not overweight, but I do sweat easily (Iraq sucked!) so this is actually a welcome change for me.

Iraq sucked a lot more for the local Iraqis who had to deal with the likes of YOU.
 
Based on my experience:

Original iPhone: uncomfortable
3G / 3GS: very comfortable
4 / 4S: comfortable (with a bumper: very comfortable)
5 / 5S: very uncomfortable
6 / 6+: haven't one yet
 
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