GSOs are certainly usually usable, unless you have bad luck and get one with lots of astigmatism (and when I say "lots", the worst one I saw had 1 wave of 'stig on the wavefront with images already severely degraded at 150x). Sadly your chance of getting a comparative lemon are larger in the larger sizes. So getting the mirror tested is a _really_ good idea, since at least you can reject anything that doesn't meet your minimum criteria, given the consumer protection laws in the EU (you might have to pay again for the testing if you decide to try a second mirror, though).
Whether you think they are "good" depends on your definition of "good". They're certainly usually not as good as my own 400mm mirror currently is (it started its life as an Oldham Optical which was decidedly "meh", with a colossal bulge on the centre and quite some astigmatism, but not enough for it to be fatally flawed, but is now refigured) and not as good as the three last Nichol mirrors I saw.
The figure can be "OK" but the RMS error of small scale roughness is frequently something which I'd guess is around 5nm on the surface (and was measured by some French -- you've been on that forum as well -- as 5-6nm as well); it's not unusable but even on a Foucault-tester you can clearly see the mirror's rough, with very characteristic striped patterns due to rapid parabolisation. You can see an example that _I_ think is a GSO (it's only opinion, or course, but there's some anecdotal experience behind it since the roughness pattern and the stress figure in the blank are _very_ recognizable) here:
That's also a mirror with a fairly OK figure but fairly rough (and with strain in the blank that makes it harder to guarantee it can be refigured easily).
So there's your example: if you think that's a "good" mirror then you should go for it -- that's what you can expect if you don't get a lemon. It will also pass the independent test quite well, since the figure's fairly OK.
I have an issue with some of the French owners who gave Teleskop Service a hard time about that roughness (it should not matter too much for planetary imaging, which was what the user said was "impossible"; my guess is that it's the GSO mirror thickness that are causing that user a headache), but they're certainly less smooth than most Synta mirrors I see.
Edited by sixela, 23 November 2017 - 05:25 PM.