I fly with them all the time as they do the best deals from UK to Poland. I couldn't get around the uk on public transport for the same money!
Crew have always been pleasant and helpful. They allow a car seat/travel cot/whatever to be checked in for £10 each way which is a bloody bargain (easyjet don't for instance). Ok it's expensive to check baggage (again the same now on most budget airlines) but they have a 10kg hand baggage allowance which is good.
As already stated on here, make sure hand luggage is the right size, don't buy food or drink on board, don't expect much leg room, you will be expected to get in your seat and be ready for departure promptly as they don't have a great deal of turnaround time.
Between DP and me we've probably flown with them about 40-50 times, never had a problem with them. (Hope I'm not speaking too soon as DP is flying into UK tonight with them!)
I've never had any extra money screwed out of me by them.
Jan 2010 tried to fly from Poland to UK, flight was cancelled due to a couple of mm of snow at Bristol (sigh), the staff were incredibly patient and helpful with rebooking/refunds for the 200 odd people.
Can't fault them.
(no, my surname not O'Leary btw!)