I disagree. Cycling has become a lot more popular, and easier, in NYC in recent years and especially in recent months since CitiBike started.
I live by 1st Ave, where there is a physically separated bike lane from Houston up to the low 40s. It's a breeze to get uptown on it, and every morning I even see young women in fashionable clothing biking on citibikes to work in midtown.
2nd Ave also has a bike lane that goes in the opposite direction but it is not separated. However there are so many cyclists on it now that cars are aware. Honestly the real danger on 2nd ave is not cars, but the potholes and other road quality issues. If you're not watching the road (literally) you can hit some nasty snags and fall.
3rd thru 5th Ave are not great. 6th Ave has a bike lane but it's not as comfortable as 1st and 2nd. Further west and you can take the Hudson River path, completely separated and along waterfront, anywhere.
The biggest hassle is lack of bicycle parking in Manhattan. That's why CitiBike has been a boon to commuting by bike. They are having their own issues with keeping up with demand and poor hardware and software but hopefully they'll smooth it out.
They are improving cycling a lot in the city and I know the paths will improve.
And a not about the crazy NYC streets. Driving in this city is not something you can do half-asleep. I find drivers are not able to let themselves get distracted because you need to always be alert. Cycling is nuts sometimes in the sense that you've got so much stimuli around you and potential hazards everywhere. The result, though, is that everyone is pretty alert and cars know you're there, and cyclists are also extra careful.
So, as far as Manhattan goes, biking is the best and often quickest way to get around, weather permitting. I don't find it unsafe unless you let your guard down or ride recklessly. You have to remain defensive and you have to stay off certain roads (I don't bike on the major streets, i.e. 14th, 23rd, Houston). You have to be assertive; you can't be afraid to yell in a driver's face when he turns and cuts off your path without looking and you have to use the bell liberally around cars.