Hi,
I bought a Celestron Maksutov Cassegrain telescope to take picture of birds. I found it to be of little use. Incrediby hard to focus and the images were pretty low contrast, but foremost, the assembly was simply not practical.
On the other hand, I have a 400/4.5 APO (from Minolta) which I use with 1.4X and 2X converters. 1.4X works with AF, but 2X does not.
Focusing manually with 2X is very difficult, partly because the focusing ring is not intended for exact manual focus. Live View (at 14x magnification) is certainly helpful.
My recent thinking is that I just put an order on a Sony Alpha 77 SLT. It's an APS-C camera with 24 MP sensor. My guess is that I won't use the 2X with that camera, because to much resolution is lost.
So to sum up: using a telescope is not without perils. Telescopes are of varying quality of course. Focusing is critical and in my view problematic.
Finding the subject and composing can be a real challenge. A geared head is probably most helpful.
Best regards
Erik
There are some good quality shots with telescopes + DSLRs
eg.

from:
http://www.birdforum.net/showthread.php?t=194789&page=6
The crispness of some of the images is like top prime teles.
Who is using this method? What is a good attachment for the telescope to DSLR? Its not clear which shots are made through the telescope eyepiece and which are other methods. If you need reach why would you go for a $6000 lens at 600mm or 800mm focal over a <$2000 digiscope setup with 400-3000mm? Other than the obvious autofocus...
Having a cheap, high quality 800 or 1000mm setup is very tempting.