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Hi Leon,
Yes, I've used one before, the FSQ-106, not the N version. The major difference being the N has a retractable due shield. Despite what many believe, the original FSQ-106N is probably still available through some US suppliers such as buytelescopes.com and astronomics.com. I know about 3 months ago you could purchase one. The prices will come down when the new Q is released.
If you intend to use it for visual use, you need to buy the extender-Q 1.6x accessory (~$380). If you simply put in a diagonal and eyepiece into the visual back you'll have difficulties reaching focus or worse, the diagonal may scratch the rear double objective (very expensive to replace). The rear object is quite close to the visual back. The extender-Q increases the focal length to ~850mm F/8 from the native F/5. If you're going to use it purely for imaging/photography, you don't need this accessory, though its nice to have to increase the focal length for some objects. The downside of the extender-Q or for that matter the new focal reducer is the image circle is reduce to 44mm which can introduce vignetting for large imaging chips. Natively in the F/5 mode the image circle is 88mm! Massive!
The new FSQ has Tak's new 10-to-1 Micro Edge focuser, as major improvement on the original. Though, I've never been a fan of manual focusing even with such fine adjustments. I prefer computer orientated focusing as the results are consistent and temperature compensated.
As Tony mentions, the FSQ has a very large flat field. No need for focal reducers or field flatteners. The Petzval design is integrated. Overall, it is an extremely efficient astrograph.
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