Online checkin has pros and cons. Since I mainly fly NW, this mainly pertains to them.

I rarely check bags, so using a kiosk in the airport is usually no slower than checking in online. The reasons I will check in online:

- 10 points in the current miles promo. Kiosk only gives 5.
- Potential to get the upgrade if EUA did not clear. There is debate in the NW forum if the online checkin actually helps on the FC standby list or if fare class is more important for battlefield upgrades.
- If flying Southwest, can get you an A boarding pass. Not sure if it affects any other carriers with a zone system.

Reasons NOT to check in online:

- Waiting until last minute to make sure they did not swap equipment and move the exit row I was sitting in (DC-9 30/40/50 and CRJ versions). If you check in 24 hours out you can't change your seat again.
- That pesky 8.5X11" piece of paper to fold, cut or tear apart.
- No computer/printer handy.

I'm not sure what you mean by checking in for a car online. I do reserve mine online, usually National, since the website knows everything about me (DL#, CC#, corp discount codes, etc.) Plus the website knows I get a full for the price of a mid, but my corporate travel site can't figure that out.