I have used Uncercover Tourist throught the mousesavers.com link as well. I think the savings is about $100-130 on a 6-7 day park hopper for my family of 4 (2 adults, 2 kids). They include tax and shipping so be sure to factor that in when you cost compare!
When posters mention that it is "safer" I think what they mean is that is is a very well known, well established business that many people have used. It is recommended by many travel guides, increasing the "legtimacy" factor. I am a pretty suspicious person and I felt totally comfortable with the process. I ordered my tickets, and they came when promised and they were just as advertised. Definitely be wary of ebay sellers - I have a friend who got burned. I am sure many are reputable, but unfortunately, one bad apple can spoil the bunch, kwim?