I've just did a few experiments with views 3 under drupal6. So far, not so good. Turns out the update issues are totally real.
Most specifically, views3 for drupal6 "loses" the pagination parameters. You can restore them manually in some cases, but if you have lots of views, this can be a chore. If you are using views for user reference, or node reference, it seems this is a total loss though: I've been left stranded with lists of 10 entries, with no apparent way to get back to the full list :( Fortunately, going back to views2 manually does work... but this does look like a major showstopper for migrating sites from d6 to d7... Pity.
