Pulled mine out tonight, (the ones from the amazon link) and here are my thoughts. It's essentially a plastic tube, about 20" long, with a 30 degree bend in it. There are three openings: one at the bottom that your hand goes in, one at the front with a drawstring, and a small hole for the eyepiece. You remove your eyecup, stretch the hole over the viewfinder, and snap the eyecup back into place. The drawstring is an interesting idea. It has a plastic 'cincher' on it to keep it in place.
The only issue I had with it was that the bulk of the string would keep the lens from zooming all the way back out (it would slip between the lens body and the part of the lens that moves and keep it from going the last couple of millimeters in). Unlike the more professional rain covers it doesn't have an opening for both hands, so you have to move the zoom ring through the plastic which prevents a smooth zoom through the whole range. The plastic isn't super thick, but it'll definitely last through multiple uses.
I had no issues fitting it to my D7000 with the 18-105 on there, so it should fit your d5100 without any issue, Pat. It was a little snug getting my hand in there, but once it was in I had enough room to work all the buttons. For $6, it's definitely worth keeping in my bag for a rainy day.