I've just bought one myself and have experienced the same kind of joy that you have with it, very much similar to getting my first iPhone.
I think your choice on iPad/Kindle will always fall down to e-ink. I get mentally exhausted at looking at backlit screens for too long, picking up the Kindle I get none of that nagging feeling of strain.
It's a wonderful device and I wish I'd got it sooner.
There's also a size/flashiness issue. If I'm going on public transit or waiting somewhere that the whole "look, I'm holding a shiny device that's easy to pawn!" thing may be a bad idea, I bring my kindle. Or if it's likely to rain or I just don't feel like dragging around the ipad and a cleaning cloth (what's with those smart cover streaks, anyways?).
Otherwise, I take my ipad. The addition of news reading (the economist and reuters apps are fantastic) is why I usually bring it instead if I can.
I think your choice on iPad/Kindle will always fall down to e-ink. I get mentally exhausted at looking at backlit screens for too long, picking up the Kindle I get none of that nagging feeling of strain.
It's a wonderful device and I wish I'd got it sooner.