There are good recruiters and bad recruiters out there. How can you tell the difference? Experience. Once a recruiter gives me bad vibes I put them on my "reply politely but non-commitally if they contact me but avoid them otherwise" list.
When you are applying to jobs, keep track of every position that you have been submitted to. This is essential.
If a recruiter which you have a positive relationship with (they have gotten you interviews, at the very least) you may want to let them know what places you have applied to, but no more than that. They need that information in order to avoid double submittal (very bad faux pas in recruiting).
If the recruiter is unknown to you or has said or done something to put you on your guard, if they ask you where you have applied to, tell them "if you would like to tell me what job you are considering submitting me for, I can tell you if I have already been submitted". Period. Be firm in your stand. If they get insistent, just get repetitive. "I'm sorry, I'm not comfortable sharing that information". If they ask why, it's just a way to get you talking and talk their way around you. Just keep repeating. Preface everything with"I'm sorry" and refuse to give them what they want. I've had to hang up on recruiters before.
You have to set your limits before you talk to the recruiters, otherwise they can make the unreasonable seem reasonable and afterwards you are kicking yourself and fretting about it, the way you seem to be now. Don't let those guys stress you out. Just make yourself a list of questions and situations that may come up, and decide what you will do. Experience will give you self confidence, and confidence gives peace of mind.
Another trick you need to be aware of is the old "can you change your resume in this way before we submit you..?" I've had several firms try that with me. The first time it happened, I was contacted by a man with a heavy Indian accent, about a position with Nike. He asked me to make some "improvements" to my resume to "showcase" my SQL optimization and normalization skills. I tried to explain that I really didn't have any to speak of. At first I thought it was a language barrier problem and he just wasn't understanding what I was trying to explain. I finally just gave up and said I would do what he wanted so I could get off the phone without being rude. The next day he called again, and again I tried to explain but he kept insisting that I just needed to highlight those skills. I finally figured out he was trying to get me to lie on my resume. I told him I wasn't interested. The next day another guy from the same agency, same thick accent, called me and asked for the same thing in a different way. I didn't waste a lot of time on that guy.
I have had a number of similar experiences that have left me deeply suspicious of overseas agencies or agencies with the word "Mind" in their name, and recruiters with heavy accents. There are enough reliable locals that I don't need to deal with overseas would-be outsourcers.
Still, for me, recruiters are essential to getting a job. There are many companies who just don't want to deal with the hassle of vetting and shuffling through reams of resumes. Also, knowing that people will often lie on or at least pad their resume, they want someone to be accountable. If they hire someone and that person turns out to be not as advertised, they can go back to the agency and complain. Reliable agencies have a vested interest in presenting a candidate honestly and companies want that.
Ask around to find out which agencies have good reputations. Do you have friends who are in the same profession? Ask them, especially if they are contractors. We end up working with a lot of different types. Try job forums...ask for recommendations on the recruiters and agencies that contact you. Join linkedin and build yourself a group of contacts. If an agency is recommended, find a linkedin contact who works for that agency and ask to network with them.
Ten years ago, job boards were the way to go when looking for a job, but nowdays they are just a place for recruiters (both good and bad) to find potential recruits. Seek the former and avoid the latter.