1: What are their transportation prices? Are they for one-way or round trips?
2: Does traffic affect the final cost? Make sure there aren't hidden fees concerning mileage, detours, rush hour, et cetera. Gas is another consideration; ask if the final bill will be per trip or per gallon.
3: How much do they charge for each person? Companies love to boast that a round trip is "only X amount of dollars!" without disclaiming that the price is per head, meaning that a family of four will face that number multiplied severely.
4: Will their shuttle services pick you up directly from the airport? Where will they meet you, at the gate or the front door?
5: Do they charge for lateness? Will you lose your ride after a certain time period? You don't want to find yourself stranded in a strange place just because you were stuck in the luggage area for an hour.
6: What if they're late? Do they offer refunds if you're kept waiting?
7: What are their drivers like? This is a subjective question, of course, but you should know in advance if they're silent chauffeurs or chatty tour-guide types. Look up reviews for the company to see what kind of reception you can expect when you land.