I actually swapped straps a couple of days ago myself. It probably took me 5-7 minutes, it's very easy to do.
Basically you take out a screw, push the bar through the strap, and take out the strap. Very easy. Just don't be afraid to give the bar a firm push or use something pointed, but blunt on the tip to push the bar through the strap. Oh and don't forget to do the same to the deployant buckle.
I would also suggest using caution when unscrewing the part that connects the strap to the watch itself as if/when the screwdriver misses the screw it will scratch the metal. I've got my own design back there now due to the misses and slip outs. Thankfully it is on the back of the watch which will generally remain unseen to the rest of the world.
As for the comfort of the composite strap, I would say it is so so. Personally my favorite rubber strap is the one from the 5066 and the 5065 generation. Those were flexible and had a very nice feel on the wrist. The composite strap, in my opinion, is not as comfortable or as flexible as the older rubber straps. Though it is fun to dunk your hand into water and see the shocked looked on people's faces as they thought that you did that to a leather strap watch...
As for the leather strap's grip, I would have to agree with you. If it was easy to adjust, it wouldn't be as much of a problem, but having to carry around a screwdriver just so you can adjust the tightness of your strap seems a bit strange.
Hope that helps.
O