Command Decision has been used in our local club for many years.
When it first appeared it was a tremendous breath of fresh air after years of complex, slow, technology-obsessed rules sets which, judging by their production values, seemed to have been produced on someones kitchen table. It seemed to lose it's way a bit in version II and these days there are other alternatives if you want a fast-moving, fun WWII game (you could look at Spearhead or Crossfire).
I'm particularly fond of the ultra-modern period (now very much out of favour) but found the spotting rules in Combined Arms to be inadequate. Modern weapons are so accurate and powerful that staying hidden is the only way to survive. The original rules were just not realistic or detailed enough. So I wrote my own.
I also amended and enhanced the aircraft & anti-aircraft rules (which were overly generous to AA weapons, particularly guns).
Both of these new rules can be downloaded below.