I've never had any dealings with anyone from ATF until today. I've read some "horror" stories on the internet, and like most things, you wonder about the credibility of the internet story.
I went to an out-of-town gun show (I know, I know, but it's something to do) and ran across a couple of ATF guys, they had a table with books and pamphlets, etc., from their agency. I decided to stop and chat with them, and I did. They were very professional, informative, knowledgeable and answered a couple of questions I had. We chatted for quite a while. I detected "zero attitude" from them about anything we discussed, they seemed happy to help and meet folks walking by. I detected no animosity toward gun shows, or gun owners, got no "trick questions" from them either.
I try to be squeaky clean and have no reason for the ATF, or anyone else, to investigate me, though I'm sure if I were trying to conduct some type of illegal gun business they would vigorously pursue their investigation of me, and I wouldn't expect that to be particularly pleasant. They'd be doing their job.
I don't know about ATF guys in different parts of the US, but these guys were fine by me.