My daughter had a bunk bed for 4 years. We passed it on to a friend's daughter a few months ago.
In any case, both my husband & I have slept on the bottom bunk many times. Not something I would want to do for many nights in a row but ok for 2 or 3 nights. To give you a better idea, I'm 5'11" and my husband 5'10. We're both 30-50 pounds overweight.