Paul, I've been playing with an MTO 10.5/1100 mirror lens, and probably agree with Dennis that images end up a little soft. I used the lens for all my eclipse shots - lovely scale! My tripod is way too light & ricketty, and anything involving any longer exposure ends up fuzzy (eg totality!). Suspect this is not only movement through the field, but vibration from shutter opening as well. It is the same as shooting through a small scope, but control of focus is much more tricky. In fact you can buy (or make) visual backs to convert the lenses into scopes! Adds a fair bit of weight if you want to piggyback too.
That having been said, it's a lot of fun, with good terrestrial applications too. Anyone got a good heavy tripod?
Cheers -