I lived out there for the 3 years preceding Sept 11th. Lived in Manhattan in Union Square 14th and Broadway. Worked out in Queens. I was in my early 30's at the time, single and I absolutely loved it.
I was lucky to be offered an Expat contract with my company, so all my living expenses (rent, bill, health insurance) were covered by the company. So the money thing was not really an issue for me, that said many of the other Brits who were out there with me managed to live in Manhattan paying their own way.
I would echo what mdb said about making friends. I found it difficult with the real New Yorkers, many lived outside the city, and I think most of my new American friends were from the West Coast who found it a much of a cultural change as myself.
Not great for biking – unless repeated laps of Central Park is your thing and even then too many runners and skaters. We use to take the LIRR out to long island to explore (still road stuff), I wasn't much of a MTBer at the time. But this then took most of the day up.
If you really enjoy city life and nightlife then there can be few places that can better NY, and it is sort of a once in a lifetime experience.
I came back because my elderly father was ill and perhaps for the last 6 months I was commuting pretty much each week between the US and UK; but I was also starting to find it a bit claustrophobic living in the city – I was either in the city or x thousand miles way either in the UK or some other American city/state. There was none of this jump in your car and in an hour or so you can be in somewhere completely different that you can get in the UK.
I am really glad that I got the opportunity to live and work there, but equally now I like my much more relaxed life in SW London. I would happily go back for a relatively short fixed term, but not for an open ended commitment like last time
