I think it's really clever and I think it works for this game. A lot of people complain right off the bat because they immediately equate it to "pay2play" just because you have a daily limit.
Really, it just works. If you use up all your allotted energy in a day and still have fun with the game, I think the developers deserve some money at that point.
$0.75 for another two days' worth of energy that never runs out is not a bad deal either.
Just wanting to throw out some positive feedback on the system because I see it getting a lot of hate in the suggestion area of the forums.
I agree, good sir! Energy definitely isn't as bad as people make out. I think one problem is some people are used to playing a game for hours on end, something that can get a little costly with Spiral Knights, as opposed to a subscription sort of thing. But I honestly like this. Helps to keep me from playing games all day :P