I guess I'm lucky. Whenever I request a block and/or the flight is not full, at check in I'm told that my seat is blocked. The only time the block doesn't work is when there's a disruption and then I'm usually upgraded to J. Case in point, out of Singapore on Sunday morning there was chaos as the inbound flight from CMB was delayed by 4 hours. So they moved me to th next flight with the seat blocked and then I was upgraded at the gate.

Most important thing is that I never expect the block or an upgrade and I'm never disappointed. If it happens then I'm a happy camper. If it doesn't then I'll get what I paid for which is a Y class seat.