This year, all as a Platinum:
- 37 CO mainline flights - Upgraded on 30 (7 were Y-ups) - 76.7% upgrade success, excluding Y-ups
- 20 NW mainline flights - Upgraded on 19 (1 was Y-up) - 94.7% upgrade success, excluding Y-up
- 57 combined CO/NW - Upgraded on 49 (8 were Y-ups) - 83.7% upgrade success, excluding Y-ups for 2007
- Longest streak of consecutive missed upgrades this year: 2
So while those upgrades can be really tough on transcon flights, for other routes, it's not so bad. Most of my missed upgrades on CO were IAH-EWR flights. My most frequent routes this year have been IAH to/from MSP, DFW, and ATL, and I've rarely missed an upgrade to/from MSP or DFW, and not missed one to/from ATL on CO.
I think a lot of it depends on where you're flying.