They'll basically service your car, top up all fluids and you're out of there in less than an hour. They ask you if you notice any problems/noises/irregularities etc.
I personally had them check the A/C because it seemed the fan never shuts off so i though the fan may be direct. But turned out there wasn't any problem. In my previous car (baleno) the fan would turn on after a few minutes, but in the city it turns on right away. :S
Im due for my free 3 month/5000kms service next week, though have driven less than 2000kms.
You're basically getting a free interior/exterior wash if you maintain the fluids yourself, but its a must to go in for the checkup to maintain your warranty