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I think the fast imaging optimized newtonians (usually F4) are ok for visual but are not the best. Short focal lengths mean a steeper light gradient needing a larger secondary mirror (more obstruction and less contrasty images), higher coma distortion (needing a corrector), are more sensitive to focus, generally need better quality eyepieces to get reasonable results and need to be accurately collimated.
Slightly longer focal length newtonians (eg F5 or F6) are better suited to visual work - they have a smaller secondary mirror (better contrast), have less problems with coma, are much easier to focus, are more tolerant of lower quality eyepieces and are more tolerant of slight errors in collimation. Of course these will have a longer focal length, so a longer tube and a narrower field of view. I am thinking of a newt in time and am looking at an F6.3 focal ratio for visual work.
Having said that, many use short focal length newtonians for visual - most of the large aperture newtonians (set up as dobs) have focal ratios well under F5 to keep the length down and make them manageable.
Just come in from a session outside - a glorious clear night down here and have managed to track down Comet Linear in Horologium.
Cheers
Steve.
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