I have a number of Tiffany gold pieces and I'm happy with most of them. I have a few of the Tiffany Paloma Picasso pieces and Elsa Peretti pieces in gold. They are well made and the 18K gold is a lovely tone.
The only problem I have had is irritation in my ears to my Picasso gold earrings (loving heart ones). I don't know why - I normally don't have sensitive ears and am currently wearing an 18K white gold pair of earrings, but for some reason, every time I wear that 18K yellow gold pair, my ears get all crusty (sorry if TMI) and irritated. I don't know if they were using a particular alloy at that time?
I have been hesitant to buy another pair of yellow gold earrings from Tiffany since. Otherwise I think they're great.