I've got one and use it all the time. I bought it more for comfort than for keeping DD away from the germs (although that's a good point too). Those seats don't look all that comfy IMO, I know I wouldn't fancy sitting in there for any lengthy period of time so I always put mine on for DD's comfort. Mine has a couple of chew toys to keep DD occupied (they are velcroed on so they can't go missing) and a little pillow can be attached (and detached) behind her back.

I love it and so does my DD. I think it was $40 at Toys R Us. The brand is Infantino.