Overall, am very pleased with iPhoto 6 -- it hasn't locked up or crashed yet, which is something v5 did almost daily for me.
[robg adds: This change wasn't discussed at the intro, and I actually asked someone on the show floor, and they replied that iPhoto still had to keep photos in its Library. Hence, I felt this worth sharing, even though it's well documented. Here's what the Help file has to say about it:
By default, iPhoto duplicates photos imported from your hard disk, leaving the originals in their original location and making a copy for use in iPhoto.Given that I long ago gave in and just started letting iPhoto organize my pics, I'm unlikely to switch back ... but this feature should be useful to those who prefer their own photo organization structure.]
However, if you already have an extensive collection of photos saved on your computer and you prefer to leave your photos in their current file structure, you can choose to have iPhoto point to your original photo files instead of duplicating them in iPhoto. Either way, you can still make full use of iPhoto.

