Couple of comments.

A-1 Limo is great (there are others). You NEED to make a reservation with most of them (not all but most) prior to your arrival. Then you call them and they give you a license plate# and you go to the curb and wait. The reason .. ORD does not have the limo-hustlers as bad as say LGA. The other issue is that there are city and suburb limo companies. The suburban ones bring locals in and out of ORD.. in order for them to go into the city ... there must be a reservation (acts like a charter). The city limos I think are more money .. and have issues going to the suburbs. Check ahead anyway!

The OTHER option if you are going to a larger hotel . is Continental Bus .. they service a bunch of hotels on regular runs. These are the larger (older) buses .. not like SuperShuttle on the west coast of the US.

Everyone touts the L as wonderful. Be aware it DOES NOT have any true luggage facilities (in that its just like the Piccadilly Line to LHR). As you get closer to the city the people crowd on and your luggage gets in the way.

A lot depends on where you are going... downtown is VERY simple on the L but most hotels then will be a short cab from a main station (not too unlike the LGW Express to Victoria then a cab to your hotel) but there are not real MAIN stations.. you have to kinda pick to one closest.

If you are going near-north (3300 North which is Lincoln Park and Wrigley field areas) the L is a pain .. as you go ALL the way downtown then have to catch another train going north (sort of like the Heathrow Express to Paddington then the Tube to Waterloo and then the Docklands Lite Rail) .. a transfer not a direct shot.

The other option (I don't know where you are going and what you are doing or where you may be staying) would be to get a hotel NEAR the airport with shuttle service to the hotel from the aiport... then come back daily taking the L downtown at that point. Some hotels will drop you at the L also.