I've used at least 16 certs in the past 12-15 months and I've had pretty good success with them, I'm in SEA and using them to ATL/DTW (occasionally MSP). They will always be waitlisted, though, I've only had 3 or so clear immediately.

Yes, occasionally it won't work and you'll sit in Y, but it's no big deal, just use it on another flight later. If you have the certs, you fly plenty of flights. Just don't get hung up on a single flight, the cert is good for one upgrade, not one upgrade on a specific flight/day.

I had so many certs this year I can't even remember the last time I was in coach... even on thanksgiving when my flights went sideways and I had to be rebooked I used a cert two hours before and cleared both segments (at the gate)