I've went through so many glass double walled tea tumblers it's not funny. They're so fragile! So I decided to go plastic. I know it's not as "healthy", but I needed something that wouldn't break after the first use.
The Tea-Mendous tumbler is made out of a durable plastic. I have not detected any plastic taste in my tea, which is awesome considering I'm pouring boiling water into it. The plastic hasn't stained at all. The seal between the double walls is effective - my tea stays warm for up to 2 hours in it.
The mesh loose-leaf tea holder is excellent - it doesn't impart a metallic taste to the tea.
The lid seals on quite tight, and I haven't had any issues with spillage. I even dropped it in my car once, onto the floorboards - and not a drop was spilled. It doesn't fit in my car cup holder too well, since it's really tall and top heavy. I would say that's the main complaint.
It would be good if I could throw it in the dishwasher, but it says not to so I don't.
All in all a great product! I would buy again.