I have used two 10" and one 8" GSO dobs, one of the 10"s being my own. Whenever I look through them, I think what increadibly good value these things are. They are great fun and allow abolute hours of deep sky observing without getting bored or running out of targets. Mirror-wise - I've so far had both the 10" and the 8" operating at around 350x a few times - that being what the atmosphere allowed at the time. The view was nice and sharp during phases of good seeing. Coma is more noticeable on the 10" as it's an f/5, where the f/6 8" is more relaxed. But the coma doesn't really bother me that much, because I'm aware of the limitations of a newt when it somes to star images in general. (I also have an apo so I know what a star looks like in an ideal world).
The 2-speed Crayfords on the modern GSOs are particularly good, although I've heard of some failing.
Both the 10" and 8" OTAs fit across the back seat of my car (one at a time obviously).
I have not seen the 12" in action yet, but going by the others, it's probably a winner too.
Having said that, it's all anecdotal evidence Duncan, which is of limited use, especially when it's positive.
Any negative reports should be more valuable because they would confirm your concerns. 
Speaking of which - there were a few
minor things I didn't like all that much:
- the secondary collimation screws ex factory (philips head) are just a pain. Replace with knurled knobs such as the ones Bintel sells.
- The primary collimation "lock" screws are in a strange position to act as such (evenly spaced between the actual collimation screws instead of near them)
- Observing height can be a bit awkward depending on the target as with any medium size dob, so that's not actually a GSO specific problem. If anything the 12" should be a bit better here.