I did in February.
It was the hardest thing I've ever done. She was essentially blind, had to wear diapers and really lost all sense of direction (she could only walk in clockwise circles... couldn't walk a straight line). Putting her down was the best thing for her, but she was my baby for fourteen years and I was a wreck for what felt like forever. It does get easier to cope after time passes, but I felt empty for the first few months.
For anyone having to face the decision, the only thing I can really recommend to you is to be in the room with your pet when you put them down. You'll regret it if you don't. It was painful and awful to see and feel the breath leave her body, but I was there for her to say goodbye. The best thing to do for them is to let them go when their time comes. You'll absolutely know when that time is.
