I think SV is a great tool for an IMC engine failure in less-than-flat terrain or with lowish ceilings, and with the EGPWS colour overlay might also add to safety in mountaious terrain when VMC.
Other than that, it is probably a matter of taste whether to have it on or off, with one exception:
On an ILS approach, I think it should be OFF. The reason for that is that you have a very tempting synthetic runway to fly to, but you should fly the needles. I found that I need to concentrate more on maintaining a proper scan with SV on - far too easy to be lazy and fly the SV display while kidding yourself that you are flying the ILS.
This is less of an issue with NPAs, where the GPS driven SV position is as accurate as or more accurate than the needles - as long as you make sure you don't bust the step altitudes. And of course on a LPV (WAAS) GPS approach the needles and SV are driven by the same source, so this is a non-issue.
If I had the half million to spend on a new aircraft with G1000, I would get it - but in my 50 or so hours behind G1000s, I always had it off so far.