I've done the Lakefront trail many times, and I'd say it is equivalent to the West Side Greenway..Both are mostly flat, both have both serious and casual riders, although the serious riders in NYC tend to use it mostly as a way to get past the traffic and not to ride fast.
If you plan on riding a nice bike to work, well, forget that plan. Use a beater bike for that unless you have secure parking.
There are a lot of places to ride, and the best is probably in New Jersey across the GWB. Either 9W to Nyack or River Road (aka Henry Hudson Pkway) to Alpine and then 9W. Brooklyn is a good place to ride, but there isn't one place to go there, except Prospect Park. Too many other places to go to describe.