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Greg,
IMHO for visual use f/7 is the sweet spot. It permits a selection of eyepieces giving a range 10:1 or a tad more between the lowest and highest magnification. What's more, at f/7 its easy to make a newtonian reflector that performs superbly across this range, and its not too hard to find excellent ED refractors close to f/7 as well around 100-120mm aperture.
For imaging... the answer depends on what you want to photograph. The Gegenschein, or Barnards Loop ? M102 ? Mars ? Moon ?
Fast Newtonians... As low-to-medium power wide-field light buckets they're fine but don't expect them to perform well at high magnifications. They are popular only because:
a) very low cost commercially, far cheaper than any other design of comparable aperture (SCT's or Maks);
b) its easy to mount fast ones at f/4 quite adequately on a base made of MDF, chipboard or plywood with little care given to the engineering of it (Meade lighbridges come to mind). Mounting an f/7 or f/10 Newtonian is more difficult mechanically and requires a much larger and heavier mount.
c) fits on the backseat of a small car,
d) the availability of mass produced thin mirrors at f/4 that are adequate optically. 30 years ago these were far more costly, hand-made one-offs and optical quality usually poor below f/4.5.
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