I'm the happy owner of a 28-300 L and I must say it's a real "allrounder".
Just browse through the database to see some results.
Most of my shots from the past year I took with this lens (I added a comment about the lens most of the time).
The main advantage of this lens is the great flexibility you get.
Shooting 300mm approach shots on one runway and 28 mm side-ons only seconds later on the taxiway without changing lenses!
Cloning dust particles away is history with this lens
1.) great zoom range
2.) good optical performance
3.) no lens changing needed
4.) it's an "L"
Of course there are some cons as well:
1.) its price
2.) you have to stop down a little more at 28mm to get best performance...from f/7.1 will do.
Nevertheless if you have $2500 to spend and you need a versatile lens, this is probably the lens you want to buy.