TAs are good to use if you don't have time or don't want to take the time to book, keep an eye out for discounts, etc. With that said, some TAs are better than others so you really have to do your homework to find one that is well versed in Disney. Most of the time any deal a TA has, you can get yourself through Disney. However, as a pp mentioned, some agencies do have blocks of rooms that they offer at deep discounts that you cannot get yourself through Disney (although sometimes dates are very limited and only a few resorts/room types are offered). One major disadvantage for me to using a TA is the TA owns your reservation until you check in, so any questions you have about the reservation or any changes you make, you have to go through the TA as Disney will not discuss anything with you. Using a TA or not using a TA is personal preference and although I book everything myself through Disney as I like to have complete control over my reservation, there are some great TAs out there that stay on top of things and apply discounts to reservation sometimes before the guest even knows there's a discount offered.