Wizzair
I use Wizzair quite often to get from Luton to Kiev and I always have the reserved seat option, each time I have flown I had the row to myself, the flights are usually pretty full but the cabin crew still will not put anyone in the other two seats. Only on one occasion has a member of cabin crew offered the empty seats for sale before the flight departed.
If you get to the gate early enough you should get the seat you want.
As gilesdavies says you should check before committing to the purchase.
Also I have personally never had a problem with their website, I understand if you use the Opera web browser it does not work correctly, which is why I use Chrome.
On another issue it will be interesting to see if Wizzair will fly from Lviv to the UK, I was talking to a member of Lviv airport managment last week who said that it has been discussed at length about a twice weekly flight to a Northern UK airport and that the airport are looking for other possible low cost airlines to serve the UK and Europe.
Lviv is only a small airport but has a huge market potential for Western Ukraine, especially when the Ukraine starts it's open skies policy.
On a personal note I hope they do start flight to a Norther UK airport, I hate the train journey to Luton.