I had a fine time with the fasttrack. It was great to be able to pay online with a credit card and they REQUIRE that you print out a copy at the end and if you are doing it by post, mail in a copy with your documents, or bring in a hard copy in person. With the fastrack I noticed it took them over 24 hours to actually deduct the fee from my account.
I did the online fastrack on a Wednesday night, on Thursday I overnight mailed my documents by Fedex, Friday morning 9am documents arrived at consulate in NYC (i love fedex because you can use their online tracking system), friday morning they also deducted the money from my account (not sure if this is a delay in processing time or if they waiting for my supporting documents), the following Thursday (a mere five business days later) at 10am I received a FedEx back from them with my documents and visa in passport.
If you go in person and don't do the fastrack you must deal with paying there in person, though i think they do take credit cards now. I just think its easier to pay online and have a nice typed application that is easy to read, so i recommend the fasttrack if you are going in person or by post.