I'll agree that the Surefire feels more like a natural part of the shotgun than just another add-on.
My experience with barrel/mag-tube mounted lights made me question their dependability. Namely, one time I found that the clamp that held a flashlight onto the mag tube was pinching it so tightly that it was restricting the flow of shells down the tube. Then, if you loosened it a little, it started to jar and move around from the recoil. Considering the nature of why I have a HD shotgun, I didn't want to risk it...I shelled out the dough for the Surefire and haven't looked back.
I haven't tried a light that mounts to a rail system on my shotgun, simply because I don't want to buy a rail for my shotgun.
As far as gripping the Surefire forend, that's not a problem. It has a naturally "grippy" rubber and some grooves and ridges that help out. It's a little bulky, but not so much that you can't get your hand around it. The only issue I might consider is that I can't but help accidentally pressing the touch pad on the right side of the forend with my fingertips everytime I pick the gun up. Not a big concern to me, but if you were aiming for stealth, a 1/5th second blast of light might give you away.