I switched last winter. It does depend massively on where you live. If I was in the Peak District it wouldn't work for me.
My 1999 Alu Cindercone built up to a 20-21lb bike (9kg) using ordinary components (P2's, Deore V's, Sugino chainset, cheap bars, seatpost etc). Losing the weight helps.
It does make you ride harder. The choice is to kick it up the hill or walk. That has translated into my road riding even, where I am more determined. SS gears are the same as around 39-21 on the road...if I can do it offroad then i can do it on tarmac :x
This made me fitter (as a 42 year old parent holding down a job maximising riding benefit is good)
You corner better - speed once lost may not be regained (especially over 22mph when I spin out) - so my cornering technique is improved.
Personally I got rid of suspension forks as I have to ride out of the saddle a lot and didn't want the things boinging and wasting effort.