We have 2 F840's in production along side 12 F760's. The big "gotcha" for us is that you cannot volcopy or Snapmirror between 5.3.6H1 and the F760 5.3.6 O/S - really not so good in our case as I would love to use volcopy to migrate from the F760 to the F840. These things scream, but its early to compare as our load ramps up. One piece of reality-versus-engineering-claims relates to disk scrubbing. We turned off disk scrub on the F760's because it really impacted user file access. I was told this issue would no longer be a problem. Well, it takes the F840's, each with 12 shelves of 36 gig drives, from 1AM to 2PM (yes, 2 PM) to scrub under very light load. Another piece of folklore I guess.
I would really like to hear where you heard that you cannot snapmirror or vol copy between 5.3.6H1 and 5.3.6. This should work just fine and we do different variations of this internally to netapp. Please let me know where you heard this so that I can find out why this rumor started and hopefully stop it from being propagated.
If it really didn't work, I would expect a bug to have been filed and I haven't seem one complaining about this situation.
Mike Federwisch Network Appliance Inc.