CG – I’ve looked at the generation times WP Cache prints at the end of each page. That’s what I base my observations on. Those times are generally the same for both engines, but I have something else up my sleeve for high-traffic sites.
If I get around to it next week I’m going to look at how well it works with WordPress MU. I think the object-cache just needs a “site_id” in the key prefix to work and even that might be optional.