In my experience, the East Side Greenway is only really useful for transportation if you're all the way uptown or all the way downtown. The greenway basically doesn't exist in midtown. You have to deal with stairs and from about 60th St. to below 40th you get re-routed to the avenues, so it's easier to just stay there. If you're way uptown, you can use the Harlem River Speedway from Dyckman St. all the way down to 155 traffic-free. Otherwise, I'd stick to the avenues on the East Side unless you're riding for leisure and looking for some nice scenery.