Very sturdy.  Super easy to put together.  The bed came in a few boxes but only had about 5 parts that needed to be put together with about 4 screws total.  I could have put it together by myself if I had to.  The bed is well made and sturdy. Perfect for our 6 year old but sturdy enough for an adult too.  
We didn't use a box spring with this bed, we just got a few more pieces of wood (it only comes with 3) to add to the bottom because the mattress we have is really tall.  If you are using a box spring with this bed, I would think your mattress would probably need to be on the thin side or it will come up really high.