Negative GoodBytes, call it what you want, but the Power Cover has issues, be it bugs OR design flaws... When it first came out, we were told it would charge whether Surface was on or off with the included instructions AND by Microsoft's own support page until we raised such a fuss over on the answers.microsoft forum they finally just changed the instructions. Error #1 Microsoft that affects all covers.
Also, per your response, it is by design that the cover will randomly drain, even with the Surface turned off, if it is still attached? Negative sir, error #2. Its not just mine with this problem, well reported over on answers.microsoft.com. On this same token, they still can't even make the Surfaces reliably sleep/hibernate/stay asleep/wake up etc.... I've discovered through regular usage the Power Cover doesn't handle these situations well and end result is a depleted power cover unexpectedly at the absolute worst time.
As for my third problem, the Power Cover is too heavy/stiff to stay well attached to the spine of the Surface without rocking around... there is some very obvious rolling around if you wiggle things at all. The downside to this is the two power plugs will occasionally lose connection and create all sorts of havoc on the system. On a solid desk in laptop mode, it will work fine, but once you start trying to use it on the go on a lap or with the power cover folded back, problems eventually arise.
It's unfortunate, and I can only assume Power Cover 2 will be better, but no matter how ya slice the bread currently, it's still stale.