I've felt dehydrated while I'm in loading phase, it's very noticeable from that perspective. But the mental effects only come after at least a week (for me: longer attention span, less impacted by poor sleep).
Well.. I can't, nobody can. Perhaps it was a placebo effect or perhaps I had low levels of creatine and felt the difference. I often felt better right after taking it though, not consistently but often.
To my knowledge creatine has no significant effects until your levels rise after, say, a week of taking daily