Demos used to be extremely important to me, and I would pretty much only buy something after I had played the demo unless it was a known property from a developer I was a huge fan of. Now a days, however, there are so many different written reviews, video reviews, previews, lets plays, etc. that if you can't get a good feel for a game without playing it then, quite frankly, you're not trying.
Is any of that as good as actually going hands on with a game? No, of course not, but it's a joke to say you have no idea how a game plays or if it's any good these days without getting your hands on it directly.