I just flew CLE-LAX 3 weeks ago. I used airmiles to secure an upgrade on the way there. On the way back, I flew LAX-IAH-CLE. The first leg they used a big plane, with the three columns of seats so I got upgraded to first no problem. But from IAH-CLE was a smaller plane (only 12 seats in first) and they made several announcements at the gate that there would be no Elite upgrades on this flight. Thing is, the flight from LAX-IAH is the longest, and the IAH-CLE was not too bad being in economy.

Frankly, I think it really boils down to what size plane they use.