We got my son a bunk bed from IKEA about 2 years ago. The mattresses aren't the standard heavy/thick ones. Think more like a futon mattress. Its still very comfortable to sleep on (I've slept on the bottom bunk mattress with my son when he was sick and slept pretty well considering its not a very thick mattress). The greatest benefit of this mattress is that its light enough to remove from the top bunk, bring down to floor level, put on new sheets and then put it back up on the top bunk. I'm short (only 5'1) and I can do this myself without any effort. I've tried the climb up to the top bunk (to retrieve toys from up there) and didn't feel very safe, given I'm an adult and weight and heights are much different for grown ups going up on the top bunk than a child.
On another note, depending on how large the childs room is, keep in mind any light fixture/ceiling fan placement in the room. Because my son's room is on the smaller side, no matter where we put the bunk bed, the top bunk was TOO CLOSE to the ceiling fan for my peace of mind, so we ended up removng the ceiling fan blades. Just something I wish we would have thought of BEFORE buying the bed for him.