Yes, twice. I inherited a Schnauzer in '02 when my grandpa died, and despite trying to make things work between Samantha and my own dogs for about 6 months (I'd been caring for her for over a year before that, but she wasn't technically mine) I placed her in a dog-savvy home with a retired couple and their rowdy poodle mix. I felt bad about doing it, but she was a bully and just could not live harmoniously with my grouchy girl and our old, feeble, passive male.
I rehomed Shiner, my Lab/Chow mix, in 2003. We had moved to Arizona and were going to be living in our RV for about a year before we moved into our new house. It was a smallish space, with 3 people, a lot of stuff to move to the new house, and two large dogs. The dogs came indoors at night and whenever it was too hot/raining/windy, but otherwise they had to be staked outside. We walked them 4-5 times a day and paid a lot of attention to them, but Shiner was totally miserable. She'd been a ranch dog for the entire 13-14ish years of her life, and a few long walks and a few games of ball every day wasn't enough to make her happy after she had been able to roam on her own land. I wound up placing her with a family with acreage. god I felt awful about letting go of her but I didn't know how long she was going to live, and I didn't want the last year of her life to be miserable.
I know that both of the dogs I rehomed did wonderfully in their new homes and were happier than they were with me, and I don't regret doing it. I feel sad that I didn't get to spend that last year with Shiner (she died shortly before I got Dakota in 2004) but I know she was content and comfortable, and that matters more than my feelings.