We used XL airways for a recent flight to Paris. Prior to the trip, I had trouble finding information about that airline. I had actually never heard of it before. The ticket was super cheap, compared to other airlines, so I was wondering if it would be a nightmare or a scam. It actually turned out fine.
If you want to reserve your seats ahead of time, you need to pay extra and do it online. I wasn't able to do it until about a month before the flight. That might actually be a limit they impose, where you can't do it more than a month in advance. I paid $40 per seat for the best "regular" (i.e. not first class) seats. I called them to see what I was getting for my $40. Their office is only open during the day on weekdays, and it took a while to reach a person. The information she gave me was actually completely bogus. She said that those seats were in a no-child zone, which turned out to be false. There were children in that area, and the stewardess had never heard of a no-child zone. I have no idea where the lady on the phone got that information.
In any case, I was still glad that I paid the $40. Those seats were in a section between the normal seats and first class, and it wasn't very crowded. We were able to spread out and lie down to get some sleep. I looked back in the cheap seats and it was packed.
You actually don't need to pay for seats at all. You can just show up at the airport on the day of the flight and they will assign whatever seats are left. If you are on a tight budget, that might be the way to go.
There was an odd issue with the luggage. Even though we were only taking carry-on sized suitcases, we had to check them. They have a 5Kg limit (about 11 pounds) for all your carry-on items combined. Considering the empty suitcase could weigh 8 pounds all by itself, that's not very realistic. I suspect that they have this policy so that people check more bags and pay more baggage fees. Your first checked bag is free, but if you had a big suitcase and a little suitcase, for example, you would need to pay for the second bag. On a normal airline, you could just take that carry-on with you on the plane for free.
Before the trip I did a little research to see if they were truly strict with this rule. I had trouble finding much information, and I did read in one place that they are strict. We decided to just check the bag and not worry about it. Since we each only had that one carry-on, we checked it for free and it wasn't a big deal. I guess it would have been a big deal if they had lost our bags, but they didn't. It all worked out fine.
The bottom line is that I has happy with what we got for that low fare, and I would fly with XL again.