We use these everyday, I have given them as Christmas presents and then was asked for more for a bday gift!
They do stand behind their guarantee. Last summer I sent back two that developed condensation inside and was sent two brand new.
I would not hesitate to purchase or gift again.
I buy them at Bed Bath and Beyond using the 20% off coupons.
I find the 12 oz Tumbler size is the best size for us.