I had to do it, because of our current landlord. I wasn't sure whether or not I was giving them up permanently, but my roommate asked that they not come along to whatever new place we get. And they do make messes of things, knocking stuff over and clawing at stuff when they want attention, etc.; it will be a lot easier to find a new place if we don't have to worry about whether or not they're pet-friendly and whatnot.
Still, I couldn't help myself. When I packed them into their little cat-carrier and drove them across the lake, they kept crying at me, and I found I was crying too. I saw them settled in, and I made sure they knew where their litterbox was in the new house. Kali actually sat by the litterbox for about half an hour because it was something familiar, and the noise she was used to was evidently comforting. (It's one of those magical self-cleaning litterboxes.) Then I had to leave before I started crying again... it was hard to drive home, because I had to keep blinking away tears.
It was hard again last night, and this morning. I've been used to the cats sleeping curled up next to me all night, so my bed felt empty and lonely without them. And then in the morning, I didn't wake up with Nuku on my pillow trying to wake me, and Kali stalking the alarm clock, like I have. And according to the people who have them now, Kali sat near the door and meowed piteously all night, waiting for me to come back... though she's settled down now, eaten and then found a little place to hide again.
I had those two since I first moved out of my parents' house... Kali was a housewarming gift my very first night alone, and Nuku was a present from friends the day after. I've been apart from them temporarily before, when they went to the vet to get spayed and once when I had to board them briefly while I was moving... that's always a little sad, but it's just temporary so it doesn't affect me too badly.
It's just hard to give them up forever, even if it's probably for the best.