My experience was very positive....
Flying Finn Air to Helsinki and back, brought my bass to Heathrow in a proper flightcase. Asked to gate-check the bass and see whether the cabin crew would let me take it into the cabin. IT IS THE CABIN CREW'S DECISION!!!!!!!! Not a decision made by a check-in clerk.
On the outward journey, both the guitarist and I were told the flight was not busy and we might stand a chance. We therefore had our flight cases tagged and proceeded to the the gate. The cabin crew didn't bat an eyelid and let me stow it in the overhead locker above my seat. (FYI... Bass flight case fits easily into overhead locker of Airbus A320!)
On the way back we were told that the flight was full, and that it was highly unlikely... Check-in clerk told us we could try our luck, but that she could not guarantee success. We made sure we were near the front of the gate queue, where we were greeted with, "ah, you must be the guitarists, you will need to ask the cabin crew..."
At this point two things seemed to help...
1) The guitarist with a short case went first.
2) We both wore ridiculous pouting smiles.
We walked aboard and we asked to leave our cases in the galley area where you enter a plane, and went to find our places. On exiting the plane we found our flight cases had been easily stored in a large locker area.
The fact I had a decent flight case reassured me that even if they refused it access onto the plane, it was going to be safe. They would take it from me as I entered the plane and return it as I left the plane, just like a pushchair.
One man's experience of flying with his prize possession with Finn Air...I'll do it exactly the same way next time!