For dSLRs (esp if you have a pro one), I did some research on this and found that for the money spent, it's not worth it. I believe that for us we are more interested in the photography part compared to the bird knowledge part (comparatively). And I believe that the fixed aperture and lack of AF do do matter and you might as well spend some more money to get a 500/4 or what and use TCs. Handling is another matter. And you know that you surely will want to upgrade your bodies in the future. You will have to get good scopes like those from swarovskis (5k + the t-mounts in), else you are better off getting something like a Canon 400/5.6 and add in a 1.7x TC and MF.
While if your investment is a p&s, and/or you are already a birding enthusiast, this is a worthile path. Coz you will be using the scope by itself too pretty often.
For me if i really want long range power and do long-range record shots most of the time (ie no diving or action stuff), maybe can go all the way and acquire a 2nd hand 8" Schmidt-Cassegrain telescope, they are not expensive at all. I believe it's 1000mm f5.6 (and remember it's fixed aperture), Wai should be able to tell you more on where to get 2nd hand and stuff. Anything bigger and it will be difficult to operate/move in the field. Optically it should be very very sharp, not sure about bokeh though.....DOF should be very thin. Not sure if you can add in a 1.4TC and get 1400/8, just have to worry about the optical result since focus is already manual in the first place. Not sure about metering, but it ain't a big deal with digital to do it manually since it the scene remains pretty constant and you just do blinking highlights review more than enough.