Well, I jumped off the loading dock at The Stan to avoid a flash bang kill one time, landed in a pile of rubbish/rubble/debris, went straight over onto my knee pads and then, as I tucked my plastic bodied AK in against my chest, used my elbow pads out in front of my body so I could roll... and come up without a mark on me. A lad beside me was very surprised and asked me a few times if I was sure I was alright; I was so surprised that I seemed to be ok that I started doubting myself! It really was pretty spectacular.
200lbs - I don't believe built in pads could have absorbed that much energy without me getting bruised or worse. Worth pointing out that I was wearing German Para Boots too, not some gucci Goretex efforts that would have probably supported my ankles about as much as a chocolate sock. Pyrohide & Kevlar gloves helped in the springing back up too - I can put my hands onto/into just about anything short of molten metal in them without any problems. I like hard knuckle gloves for the look and better feel through the fingertips, but compared to my Pyrohide ones they're shite for protection.
I suppose it depends on how physical you want to be on an airsoft site, but I also enjoy being able to slosh through mud/water because my boots can take it and kneel in medium consistency mud because my knee pads are big enough to keep the damp off my kecks. Now obviously if I were being fired at with live rounds I would take those small discomforts in my stride and, indeed, there are times when airsoft gets my adrenaline going enough that I will ignore wet/dirt/thorns/etc, but nonetheless it's nice to have a head start.
What fantastic knee pads do I refer to then?

ACM £12 for knee and elbow pads - 3 times through the washer and they started falling apart so I fixed them with black gaffer tape. Since then they've had a layer of WebTex camo tape on them too, although it's badly worn through on the knees now. The point though is that they retain the fat foam pad behind the wide hard shell. The same with the elbow pads, although I have replaced them with some Hatch Centurion simply because I managed to get 'em for £8-ish.
I can't imagine choosing built in pads unless i spot a monster deal so it's worth giving them a go because I don't care if it turns out to be a waste of dosh