I have booked two cruises on line. One is a cruisemates cruise coming up next week. Our second cruise booked online came from the same agency cruisemates is using for our Amsterdam cruise. We shopped around and got the very best rate from them. So far so good. Not tons of service, but our credit card was charged to the cruiseline which is a very good sign that they're on the up and up. We have our cruise docs for the cruise next week and they arrived in a timely fashion. Since we are experienced cruisers we didn't need a whole lot of service or hand holding. Any questions we asked were answered. Not too much telephone support but e-mails were answered promptly.
Booked my very first cruise with a local TA...Since then we have booked every cruise with an online TA. Since most of the TA's we run into in chat and on the boards specialize in cruising, the value they offer usually exceeds what you can get with just a local TA. Have used several different ones and have had no complaints about service.