Just like practically anything else in life, it's "not for everyone." In the case of straight people, women have a far easier time since they are the ones being constantly pursued.

As soon as it's not fun anymore, someone who is using it should stop using it.

Any straight male should have his expectations managed before starting.

General Expectation Management

• The women are not going to "fall in your lap" unless you are absolutely movie-star attractive.
• You are going to run into rude people, flaky people and liars.
• You are going to have to exert a lot of effort for seemingly very little gain.
• If you are in your mid-twenties, for heaven's sake don't act super-serious either in your profile or your messages. Let any serious relationship develop on its own, offline.
• If you live near but not in a big urban area (Chicago, Boston, LA, DC, etc.) you may have your chances shot to hell because "no one likes a suburbanite" (depends on state of public transport, whether she owns a car, but I've been shot down twice for it specifically).
• As a corollary, OKCupid works best in or near large urban areas.
• Women can be even shallower than usual (not to say that men aren't), but due to dynamics for straight people (women always being the pursued), "a 5 can have the attitude of a 10 and get away with it."
• Don't judge a woman by one picture you dislike. In fact have a friend who deliberately made her primary a not-so-great shot to dissuade shallow men.
• Not messaging her SOLELY because she's very attractive is the coward's way out.

Notes from personal experience

• May/June is terrible if you're mid-twenties because many of your desirables (regardless of orientation or sex) are still in school. They get busy and you may be forgotten in lieu of that bio test.
• (Back to straight-male oriented advice) If she says "It was nice meeting you" you're screwed.
• Not every woman without pictures is hideous.
• If your response rate becomes good (over 10%), watch yourself especially if your area is sparsely populated. Why? No one likes being second-fiddle and if you hit it off with one you may "lose" any chance with the others should the first flame out on short notice. So as manly as you may feel for chatting with ten women at once, it may be an unwise move in the long run. Let things play out.
• If she rates you highly, you should message her immediately assuming you also find her attractive on all necessary levels. This is how I found my first OKC-spawned relationship (short-lived though it was).
• When viewing profiles, be sure to drop by the "She cares about" and "Unacceptable answers" tabs. Because people don't always "say what they mean" in terms of how important these questions are, it's valuable to see if there are any deal-breakers lurking in either section.
• Use the site long enough and you will get a sense of whether she is likely to respond/worth messaging. You will start to pick up on idiosyncrasies and "catch-phrases" in profiles.