God the amount of corporate apologists on here is ridiculous.
It is always ok, as long once you see that it works you delete it, and go buy a legit copy.
Demos are not always available, and what's more they are becoming rarer and rarer. A greedy publisher won't want to risk turning people away from their crap product with a free demo, so why would they bother making one? Along with that, most reviews nowadays are bought and paid for, so who the hell can you trust? You could try looking at the user reviews and try and find a legit review amongst all the trolls and fanboys, but good luck with that.
The fact of the matter is that publishers brought this upon themselves with their own greed. If they don't want people to pirate their products for a test then they should release a demo. It might be expensive, but I'm sure they can take some money away from the ridiculous amount marketing gets.
Every other product either lets you have a free trial or will give you a refund if said product doesn't work. Why the hell should games be any different?