I like Castle Siege. The variety of layouts give us an advantage, since most of kirby's game is aerial. Other opponents have to adapt their games to the constantly-changing layouts so they can find an advantage, where we just continue with whatever we like to do.
I'm not saying I like the stage for the varying layouts, just that it's one of the pro's. Another one could be, me having a taste for platformed stages, the many platforms that interrupt the opponents. A nice kirby would easily use those to his advantage against any character he desires, obviously while playing smart. And 1/3 of the layouts is a flat stage that can be used for easy gimping... Which is what we love.
It's not my favorite stage, but it's not one I hate, either. I actually like playing there because it hinders others' characters more than mine, giving me an actual advantage, if the matchup is a difficult one, and the opponent hasn't studied the stage as much as I have... But, of course, difficult matches where counterpicking stages wouldn't help much, I wouldn't want to bring up this stage... Just use it if it's CP'd against you, or if you know you'll win for a reason or another.