Yeah, I use one... but it doesn't seem popular anymore. I picked mine up (in titanium!) before I knew what I was doing. It is a good cross between a cup and a bowl, but like many multi-use items it doesn't perform either task particularly well. If I am splitting a freeze dried meal with someone its good for eating out of. It also works for hot cocoa/tea, though liquids do splash out rather easily. I usually backpack with others where we have one pot for cooking/heating water and then need a separate container to eat/drink out of. In that situation a Sierra Cup makes a reasonable choice.

A bowl might work better in this situation. It would be easier to eat out of a bowl and more vertical walls would probably hold liquids better, but the handle is good for not burning your hands. A cup would have a handle and be much easier to drink out of, but a bit more difficult to eat out of. It is a trade-off and I am not sure what I would choose if I had it to do over. In July my wife is going on her first backpacking trip with me. I bought her a Ti mug and will have a more direct comparison then.