Post-graduate work is useful if you want to specialise in something or fill-in gaps. The only post-grad work I've done is a Diploma in Business (essentially the first half of the MBA programme there), which was good to add a basic commerce background to a computer science degree. I think that I got quite a bit of mileage from doing this, although the MBA lark is quite hard yakka when you're also working full-time. I may possibly go back and finish the MBA, but I think that's really getting into diminishing returns and the effort would be better spent elsewhere.
I may also fill in some more maths at some point or do something in Actuarial Science (I'm currently working in Insurance and getting substantial career mileage from this industry specialisation). These really have the effect of expanding outwards from my base in Computer Science. To take Computer Science further I might do a postgraduate qualification in a field that interests me, perhaps Database Systems Architecture.
The key here is interest and relevance. There is absolutely no point in doing a postgraduate qualification that doesn't interest you. You won't enjoy it, you won't get really good results and you will spend a lot of money and waste a lot of time without really enhancing your career.
In a postgrad (or any student for that matter) I'm really looking for someone that gets at least some A's. This shows that you found something (preferrably a non-trivially difficult paper) that you were good at. A masters full of B's and C's with no good grades looks a bit odd.