I've had good luck getting (semi) reasonable upgrade offers at the gate on both United and Delta international flights. But not on Continental yet.
Often the checkin process or agent doesn't automatically offer, so I always ask to see what's on offer.
I like having this option at flight time. Sometimes a little extra sleep may be worth it, sometimes not. For me it would be a positive step to see CO follow UA in this regard, rather than send out empty seats...