In reply to RocknRoll:
My brother's Sigg has green mineral growths in it ... leastways, I think they're mineral growths. If they're living, they have resisted attack from lemon juice and bicarb. He still drinks from it. =S
My slightly more venerable Sigg has aquired an interesting flavour, but I rather like it. I try to keep it clean and replaced the cap when it went black around the seal. Nowadays I'm more worried the metal's going to puncture ... there are some rather large dents in it.