I have 4 cats that are all able to use this thing at the exact some time.  My two boys are both 20 plus pounds and are able to use this thing with ease and it's sturdy enough to hold them without teetering or sounding like it will fall apart.
The only downside to this, is I was not expecting the very top "tunnel" to be as small as it is. None of my cats will even attempt to go in it since it is so high up and small.