Their call centre charges are a rip-off. Only call them if you MUST, because the price per minute is very high, they don't speak English very well, and you'll be charged £15/EUR if it turns out your change could've been done by yourself online.
Have flown over 25 times with Wizzair and almost, almost, had a good or neutral experience every time. You have to make sure your hand luggage is the correct size, bear in mind that normal-sized, 10kg cabin bags now MUST be paid for as checked-in luggage, and consider getting priority boarding as it's worth it.
Problems tend to be more with your fellow passengers, frankly - the old Kosice-Doncaster/Sheffield route (similar to a lot of Romania-Britain routes) was quite unpleasant due to bad behaviour and, ahem, hygiene issues. Also, the on-board selection for food and drink isn't so good. Finally, many flights from Luton are crazily early.
Still, better than Ryanair!