I JUST switched to Aperture from iPhoto. To be honest, iPhoto was working great for me for photo organization but I never used it for photo editing (except maybe fixing a red eye here and there). But, the most recent version of iPhoto (which looks really pretty and does some things really well) completely messed up my photo tagging system, so I reinstalled my older version until I could make the switch to Aperture.
So, Friday was when I installed Aperture, and I spent a lot of time yesterday getting all my keywords organized and making sure my iPhoto library imported properly. And, now I'm ready to get going. Just from playing around, I am really liking their Adjustment controls for photo editing. I know Lynnette also uses Aperture, and she's been using it for a little while, so she probably has some good tips.
I don't think you would ever regret getting Lightroom. I've only heard great things about it. But, it is much more pricey than Aperture, so that's something you have to weigh.
I'll be watching this thread to see what pointers others might have for using Aperture.
ETA: If you have been using LR for photo editing, you will not be happy with iPhoto. It will not be nearly as powerful as what you are used to. It should work fine for organization though.