If your luggage is still going to be forwarded on without any problems, then you might as well do something outside the airport. Obviously if your luggage will no longer be forwarded on (i.e. you have to check it back in again before your connecting flight) because of the lengthy time now between flights, it would less feasible. I'm sure you could get a taxi and get the driver to drive around a bit... could get expensive, though!

You will need to collect your luggage in order to go through Customs at O'Hare- then as you come out of customs you can deposit it for onward flights. Is your connecting flight on the same airline?
Allow an hour or so to get through Customs and Immigration and then you can set off. If you walk out any of the doors marked 'Ground Transportation' you will find outside several shuttle services- Mears Shuttle, Supershuttle, there are many. For around $19 per person you get a shared van ride to any of the downtown hotels. You may have to wait for him to fill up with passengers, and you might go to a couple of hotels on the way,but this is the cheapest if you are just one passenger. Just ask him to drop you at the Swissotel on E. Wacker Drive- (I just give that hotel as an example,if you have another in mind...ok) Once you get there- if you're facing the hotel front door, then walk down the hill to your right towards the bridge (1 minute)and then walk to the right (NORTH) crossing the bridge- now you are on North Michigan Avenue (the Miracle Mile)which has everything you need to amuse you for a couple of hours or more. Shops/restaurants,skyscrapers to gawp at, go up the Water Tower for great views over Chicago - the Chicago river for a quick boat ride, or the Lake (Michigan) to wander around. Take the FREE trolley ride up and down the avenue- look for the sign in the front window that indicates it is the free one- there are some that charge for a tour.

If there's more than one passenger I would just get a cab from O'Hare, ask him to drop you anywhere on NORTH Michigan Avenue- (you don't want South) oh, outside Virgin Megastore or Saks will be central, it is a long long road. Taxi runs around $40 plus.
I would be sure to get a taxi back- you don't want to wait around for shuttle services- taxis are plentiful,just raise your hand and hail a cab on the street-yellow cabs are everywhere....do not need to book ... .Need to be back allowing plenty of time to get through traffic- O'Hare is about 15 miles or so outside town- then of course Security screening etc- depending on time and day it gets crowded,you probably know that
You can also get a subway train, for around $3 per person each way, but honestly I wouldn't mess with it in case of confusion with stops, delays etc,given that your visit is very limited.
Chicago is a fabulous town, hope you get to see some of it.

Patkin- I just looked at your travel date- that would be a wonderful time to see Chicago at it's best with all the Christmas decorations,shop windows etc- have a look at Marshall Fields (State St), Nordstroms, Macy's .... no expense spared. If toy shopping then FAO Schwartz.
Google a map of the area so you know your way around before you start.

Thank you emdia43 and greyeject for your replys.
I'm sure Chicago is a lovely city but we've decided just to bide our time at O'Hare airport. 8 hours is a long time to fill in but knowing my luck some unforseen incident may lead us to be stranded at Chicago, and i don't want to miss our connection to Vegas...

I don't know about O'Hare, but quite a lot of large airports have airport hotels attached to the terminal where you can rent rooms for a day or part of a day. We did this at Orlando airport once. You can use the room to rest and freshen up and also use the leisure facilities, such as the pool or the gym. The hotel we used had live flight arrivals and departures on the tv in the bedrooms.

No problem, my luck is the same....
Check into whether your airline has a travelers club, some of them will let you use their club facilities for a day for a 'small' fee,at least you can wait in comfort then.

Downtown Chicago is at least 30 minutes from O'Hare and more if there is traffic. Take the travelling out of the 8 hours plus the unloading and checking it times and there won't be much time left to sight see! taxi fare would be $40 to $50 each way.