Hi David,
I have the 70-200 f/4L IS, and the 1.4x Extender. The 70-200 is wonderful, absolutely tack sharp and really easy to use. IS is a personal choice, but I love this too. The 1.4x Extender works really well, and doesn't degade the images noticeably. I do find extenders to be a bit of a pain to use, swapping on and off etc, but they are a good compromise.
I'm tossing up what longer lens to get also, as you can read in a thread started a couple of days ago here:
http://www.iceinspace.com.au/forum/s...ad.php?t=70581
Cheers,
Jason.