Mousesavers doesn't directly sell tickets -- they just recommend the best deals out there. It's definitely worth it to buy tickets in advance, but few vendors do much better than buying online from disneyland.com. You can check prices at getawaytoday.com and lastminutetravel.com, but you'll probably only save a small amount. The advantage of buying directly from Disney is that you can print out the voucher at home, and bring it directly to the turnstiles, bypassing the ticket windows. There, they'll scan it, let you in, and exchange it for the same credit-card sized Parkhopper that you'd get at the ticket window. Takes no time at all.
This can be a large advantage on the mornings of those days when the ticket windows back up. AFAIK, all 3rd party vouchers must be exchanged at the ticket window, costing considerable time. It would be worth it to me to buy from Disney online; your mileage may vary.
The one exception to the above rule is AAA. It also has to be redeemed at the ticket window. And the price is no better than Disney's. What makes it worth considering is that (last I looked), they include free parking with your Parkhopper. That's worth $15 a day, if you're driving.
HTH,
Bob