I am so far from a coffee mug expert, but this GSI infinity backpacker mug has been awesome for me for the last couple years, and I haven't seen it mentioned much here, so I thought I'd give it a shout.
I know, strictly speaking you could just not carry a mug, or use something lighter, but there are two reasons this mug stands out as incredibly useful and totally worth the 3.5 oz.
1) You can close it and it'll stay closed. So you basically gain the functionality of having an extra water bottle (17oz ain't bad) for sipping on coffee, water, or a sports drink on the go, or as a food container for storing leftovers. Its a really tight seal, and I've never had an issue with it opening even when upside down, but its also just a quick flip to sip from it. The strap sewn to the cozy also makes it easy to latch to my HMG ice pack (no pockets).
2) It insulates well for the weight. On cold mornings it doesn't take long for a cup of coffee to cool in an open or uninsulated mug. With other mugs, its usually cold by the time I finish it. With this one, I can close it and forget about it while I take down camp, and its still at a good drinking temp when I come back to it. Same goes for food if I don't want to throw a whole meal down at once. It also beats eating out of a bag.
For $10 its easily my favorite inexpensive piece of gear. Super functional.
http://www.rei.com/product/874270/gsi-outdoors-infinity-backpacker-mug
