Buyers Broker

Positives: it's free for you; a really good one can help you navigate the coop-board approval maze
Negatives: most will get on your nerves to such a great extent that you'll wind up either homicidal or suicidal ... or both!

type in the search discussions box "buyers broker" with quotes to see numerous discussions on the topic.

to start you off
http://streeteasy.com/nyc/talk/discussion/14760-is-thre-any-reason-not-to-get-a-buyers-agent

I'd rather deal with the listed broker. You will get direct and better feed back and if you're in a competitive situation you will have an advantage because the commission will not be split, regardless of what they tell you.

Only reason to use buyers broker is if you are unsure of what you're doing. I've gone out both ways and although still haven't bot anything, that's how I feel.