If it doesn't work (crashes constantly or advertised features are missing/don't work) then you should have no problem getting a refund. If you just don't like it, you might get lucky, but you might be refused - it's down to the individual agent who handles your case.
I really think the app store should introduce the Android system for purchased apps - you buy it, and if you decide within 15 minutes that you don't like it, go back to the app market, hit refund, and the app is deleted and you aren't charged. I'm a bit of a tightwad when it comes to paying for apps, and I often don't by apps in case they turn out disappointing and I feel like I wasted my money. If apple introduced that system, I would buy a lot more apps (and probably end up keeping the vast majority). That way I would end up with lots more cool apps, developers would get more revenue and continue producing high quality apps, and of course Apple would get its 30%.