I must be lucky, out of 32 flights with them last year none of them were late or in any way eventful.
The boarding process is a painful experience I'll admit, this is especially true at Luton with the stairway holding area which becomes seriously uncomfortable after 20 minutes of waiting either in the boiling heat or freezing cold. It's not uncommon for them to call for boarding while the inbound aircraft is still over half an hour away. In most cases half the passengers are processed before the holding pen/stairway becomes full and those with priority boarding are pushed up against the fire exit and windows. Other airlines are guilty of this process but Wizz appear the worst offenders.
The actual physical boarding of the aircraft always seems very orderly, there's always enough room in the overhead lockers, passengers take their seats quickly and there's very little seat swapping or lingering in the aisle, it's usually a far better experience than Ryanair or other low cost carriers of similar calibre. I've never had a problem with crew or their professionalism either, most of my flights are operated by Lithuanian or Polish based crews so are well established with Wizz and have plenty of experience at this point but I'd hazard a guess that Wizz UK or staff at newer bases are inexperienced, rushed and potentially trained with a "learn on the job" attitude thrusted upon them.
All flights on time, crew are usually well groomed and professional but not exactly friendly, flight crew informative and aircraft are spotless and comfortable. Experience on the ground varies from average to horrendous depending on the handler, Luton and Budapest being horrendous and Vilnius being average.