WE arrive from the west coast at 3pm and our flight to vienna isn't till 10pm (frequent flyer miles- beggars can't be choosers). Any thoughts on what to do? Is it worth it to go to NYCity? what would be the best way?

I think it will depend on what day you are traveling through. Is it on a holiday? Is it a weekend? I think NYC is always fun, and you would have time to go in to the City - but you would have to allow enough of a buffer that if there were traffic delays (like if it were a holiday and there were parades or other nonsense going on) you could make it back to the airport. Also you would have your hand luggage to deal with. You could do any number of things: see Times Square or go to Central Park, or go online and see what is going on in the city that day. You could go up the Empire State Building...the list is endless.

Is it worth it? You bet.
I can't imagine spending 7 hours sitting in JFK doing nothing.

There is a shuttle for 15 bucks that will take you straight to Grand Central station. There you can wander around, see the city, get some great eats,(we love the Oyster bar in Grand Central, pricey but great) and then jump back on the shuttle which picks you up same spot and takes you back to JFK.

a cab will cost about 45$ each way.

The AirTrain from JFK stops at both subway stations at Sutphin and Howard Beach. From there you can take the subway to go to, say midtown Manhattan. It takes a little over 1 hour to do this.

Hence, considering a 2 1/2 hour of subway and AirTran round-trip travel time plus a 1 1/2 hour pre-boarding wait at JFK for you flight, you have about 3 hours to kill in NYC. An early dinner in midtown Manhattan would be your best bet. The E train from Sutphin stops all at midtown East, midtown West and Times Square areas. You can get off at Times Square, soak in the lights and ambiance at Times Square and Broadway, grab a quick dinner and get back on the subway and AirTrain to JFK.

Or, as is customary for us before we go on extended vacation to Europe, you may want to grab some Japanese or Chinese food (as sort of pre-cursor to all that European food you'll be having in Europe) at 53rd/Lexington station in midtown East Side ( this is the first station in Manhattan after Queens). There are a lot of decent Japanese and Chinese restaurants in the area.

One way cost of AirTrain is $5 and subway is still at $2 (as I write this).

Your biggest obstacle is your luggage if it's not checked through to your final destination. Due to security issues, it may not be, even if you're staying on the same airline. So if you can check in early for your ongoing flight, get your boarding pass and get rid of the luggage, that would be terrific. Then as others say, your choice is a cab for $55-60 total, a bus to Grand Central for $15/person, AirTrain/LIRR for $12/person or AirTrain/subway for $7/person. The trip is about an hour.

If you know what you'd like to do or see we can help you get there.

One of my favorite activities in Manhattan (we live only an hour away by train) is Chinatown. My wife and kids do a little shopping, buying some knock-off purses, jewelry, etc., then a great meal. From Grand Central or Times Square, it's easy to get there by subway. From Grand Central, take the 6 line. From T. Sq. area, the 1,A,C,E,N,Q, or R and get off at Canal St. and walk a few blocks east. Subway is $2 each way.

If you plan on taking the subway from the airport and make a few stops, you may want to get a Fun Pass. It's $7 and allows unlimited trips on the subway for 1 day.

we are coming in at UNited- where would we pick up the shuttle/ and or the air train?

The AirTrain has stops at every terminal I think. It is now used as internal transport as well. There should be signs everywhere.

I would definitely suggest using the Airtrain,as you wouldn't have to worry about traffic if you take it,which could eat up a bit of time if you take a cab/shuttle.