When I booked our upcoming trip a few weeks ago, I happened to get a very knowledgable CM at WDTC. I was asking her about booking on line vs. over the phone. She had told me that there are so many mistakes made by booking on line. There's nothing better than speaking to a real live person. I llike having my reservation e mailed or faxed to me so I can see it. Also you get so much more info by speaking to someone, you can book your room, ask for codes (or at least the CM I spoke to said so) ask about plans and any money saved and you can book your PS (if within time frame) all in 1 phone call. I think it is very important to get a name of the person you spoke to. I have found some CM's to be great and some just terrible, but a TA has to call the same people so you are not doing anything diferent than a TA.
Now about using a TA. There are pros and cons. I personaly do not use one, because I think it's crazy to call a TA who will in turn call the WDTC (the same people you would speak to) to make your arrangements. When I speak to them I have so many different questions once I start the booking process. By booking thru a TA, you give up control fo your booking. If you have a question or a change, you must call the TA. I also like having the documents mailed directly to me instead of going to the TA's office then to me.
Some people swear by a TA because of the codes, but I have asked the CM's directly over the phone for codes and they ahve applied them. You can also find out about codes here on the DIS.
This is just my opinion and what works best for me. Once you get opinions from both sides you can figure what's best for you.