My 2 cents worth:
I have non-\revving Delta as OAL (ZED hi) for 4 years now, even before NW was a gleam in Delta's eye, and I have to concur that they are the best. They always upgrade when they can (over 50% of the time).

As for ZED ticket interchangeability: Ultimately it comes down to the gate or ticket agent. Lots of ink has been spilt all over this forum and others on this subject, and I don't think there's a universal set of rules. In theory, I used to think, you should be able to use a ZED ticket in a particular price range (Lo, MED, HI) and a particular distance range (Zone, that's where the Z comes from) on any flight , any airline, in that distance range and if it's in the price range as defined by your airline's OAL agreement. I put it to the test early on and it worked between EVA and China airlines (both Taiwanese). But I have never put this to the test since, and always ended up buying more ZED tickets than needed. Specifically, I have heard many times that the ZED tickets must have the correct city pair, so the interchangeability based on zones is suspect (again, up to the agent, but who wants to run that last-minute risk?).

The "date" part of your question is definitely OK: you can use a ZED ticket anytime (up to the expiration, 3 or 6 months in our case I think)

Unfortunately, ZEDs are a bigger issue for me now, since the NW-DL merger has meant that I as a parent no longer have the great "own airline" priveleges with DL I had with NW..ALL our flights are ZED now. So a transcontinental one-way flight has just gone from $50 to $140, and so I'm flying revenue in August. Overseas nonrev trips wll probably still be a good deal, and we will probably non-rev back to southern France in October.

Good luck!