Product review (5 stars): Great Item
Our old stopper would let water slowly seep out so I was hunting for a replacement when I spotted this.
It's a great little tool.  You can leave it in the sink to keep silverware or other items from slipping into the disposal and it really does seal the drain when you push it down.

My only complaint would be how difficult it was to initially put the rubber rings on, but you only have to do that once.
Handy, multi-purpose and inexpensive.