The guys I know who used a prep service appeared confident and afterwards seemed somewhat relieved by being "over-prepped". At least two guys who I think didn't use a prep service looked like they were going to barf the whole time from stress, even though their backgrounds were at least as good as anyone else's. A couple started off cool and collected, and seemed to get anxious as the day progressed.

At an interview I did some time ago, one candidate showed up with a logbook summary instead of a full logbook. Instant self-induced interview failure that probably would have been caught by a prep service. What's avoiding a 6-12 month re-attempt wait worth to you?

Some people get one shot at this, and some people just aren't naturally good about talking about themselves. Arriving relaxed and confident that you've done all you can do seems to be a good approach for a lot of people. Some people show up cold, and just naturally knock it out of the park. I don't think too many people can do that.