It depends. When "Unlimited" service is sold with a Terms of Service that includes an "Excessive Use Policy" like Mozy's [1], the heavy user is operating in an ambiguous zone where continued use of the service at the advertised price is at the whim of the provider.

We've seen this with unlimited data plans with the cable ISPs; they're not really unlimited.

To me, it's similar to having a traditional credit card versus one with no pre-set spending limit. Which would you rather have? The latter has a limit, you just don't know what it is until you hit it.

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[1] http://mozy.com/terms/ "....excessive use of the Service, which means usage over a given period far exceeds the average level of usage by users of the Service generally..."