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I would have loved to use an OAG on my Newt years ago, but backfocus becomes a killer without high quality thin OAG and correct fittings. You also have more weight coming perpendicular from the scope which can be an issue depending on focuser and tube strength around the focuser.
For refractors, you usually have closer in centre of gravity with OAG. For my chinese mounts (EQ6, HEQ5) get far better stars (eccentricity, FWHM) than through a guidescope (which I thought was reasonably well attached). Pain not being able to easily use Sharpcap though.
I'd hate to be having to reliably find guidestars through OAG over 1.5m FL - unless you put a rotator into the mix - then more weight, fittings, complexity.
Big variations in quality/tightness of OAGs too between models/brands.
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