Not leak proof but very good mug!
The fairshare mug is great but not leak proof! I've returned it thinking it was a junk one but got the same problem with the new one.
I like it because it fits my whole cooking set in it. Although the fuel bottle needs to be very small in order to fit it all.
Im giving this mug a 4/5 stars just because it leaks when the lid is fully closed. It's not huge but if the mug does tip over, you'll get drips coming out!
Buy this, it's a great mug overall : light, and convenient!