What are the pros and cons to using a 2x extender?
Using 70-200 F4 Canon L series lens and Canon 40D.
Thanks, Eric

Here is a link with your lens mentioned.

http://www.the-digital-picture.com/reviews/Canon-Extender-EF-2x-II-Teleconverter-Review.aspx

Cons: the lens will not auto focus, picture is not as sharp.

Some are saying it's best to buy a 70-200 F4 Canon with a 2x teleconverter than a 120-400 F5,6 Sigma.
Your thoughts?

I bought the Sigma 2X teleconverter for their 150mm Macro and can't really say I was pleased with the result. It does eliminate autofocus, but it also seemed to move the focus point behind what was in focus in the viewfinder. Very unnerving and difficult to use. My rate of kept shots is very low using that device. I bought it mainly to reach 2:1 in my macrophotography, but it also produces erratic results as a telephoto aid. Not quite the magic bullet I was hoping for, but I did buy it knowing they were problematic.

Decided on Canon 70-200 F4 non IS with 1.4 extender.

So, we are waiting your first uploads with it :-)

waiting for tax return.

Doesn't that 1.4X extender only give you another 80mm extension?

I use an Olympus T-Con 17 It's a 1.7 telecoverter the only one i've found to be better is the Nikon TC-E17ED but good luck trying to find one of those. It has a 55mm thread so sometimes you need a stepup or stepdown adapter to fit it on your lens. My lens zooms to 504mm and with telecoverter attached it zooms to 856.8MM and the autofocus works. Great for closeup bird shots

I think I should rethink my plan, maybe go for a 100-400L and forget the extender. 80 mm is not worth the money.

Don't change your mind Eric. The 70-200 with 1.4 converter is a winning combination. Check this - Flying High. Taken with a 70-200 F/4 and 1.4 teleconverter