I bought one of these and got it on an EasyJet flight this weekend.
It does measure about 55x40x20 unpacked, but... It has space to expand... By the time I'd packed as much as I could get in and used the straps on the side to squash it, it was up to 25cm in depth and was stretching over the 40cm too (Finished up about 55x44x25).
The check-in lady was convinced that it would be to big, but my wife squeezed it into the guide tray perfectly.
The material is tough, but not too rigid, so you can squeeze it into tight spaces.
I like the side straps, as they help to keep to 25cm measurement in place.
So... if you wanted it for Ryanair, you would have to be careful not to overfill it, but for EasyJet it is perfect and pretty much fills to the max possible dimensions.