RPS wrote:
What is the maximum telephoto length you would be comfortable shooting
handheld, with IS as well as steadying your arm with elbow on a railing
etc, but no tripod? Thanks.

As others have said, it depends on light level.
Also, as focal length goes up and/or exposure time
lengthens, the percentage of sharp images decreases.
So try multiple images when pushing limits.

Some examples:

1120 mm focal length (35mm equivalent) 1/3000 sec, IS:
http://www.clarkvision.com/galleries...wk.b-600..html

650 mm focal length (35mm equivalent), 1/1600 sec, IS, very sharp:
http://www.clarkvision.com/galleries...598.b-700.html

On a support, but 1/10 sec at 1300 mm equivalent, IS;
support moving from a vehicle:
http://www.clarkvision.com/galleries...1500b-700.html

Roger