We bought bunk beds when we found out we were having our 4th child- a surprise:) We have a four bedroom house, so we had to move our youngest son in with his brother. What I liked about the bunk bed was that it took up less space in the room than 2 beds would. Really that's it.
I wound up HATING them. I thought they were a royal pain every time I had to put a fresh mattress pad, sheets, and blankets on the top OR bottom.
When my sons got stomach bugs in the middle of the night, and threw up all over themselves (1 one night, the other the following night) and their bedding....well, I'm sure you can imagine- not fun. If the child on top was sick and needed to have his temp taken or needed meds in the middle of the night, it was less than ideal.
We finally wound up separating the bunk into 2 beds. Yes, it took up more floor space, but visually, the room actually looked bigger because the bunk bed, because of its height, was kind of like a wall and made the room seem smaller and darker. I was a little surprised that the boys were happy when we suggested it. Turned out neither one of them liked sleeping in it- climbing on it, yes. We had a rule against climbing, but we found they would sometimes do it anyway when we weren't looking. That, of course, made me nervous.
So, unless I really had no other way around it, I'd never do it again. If we're surprised again.....we're moving to a bigger house :)