iPhoto is pretty good. I'd have to say I prefer Aperture though (granted it is expensive if you let that stop you

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The lack of any zoom function within a normal viewer of iPhoto window is quite annoying. I normally find myself wanting to zoom in on the 10 glorious megapixels of photo detail, but I am instead forced to "edit" the photo, which is annoying when it takes a second to load and further shrinks the photo on the little 13" screen of my MB... The simple addition of the Loupe in Aperture makes things positively wonderful when I am, say, examining just what sort of critter I happened to photograph from the next peak over

(mind you that is without any crazy zoom lens. I love camera technology, lol)
Really, I kind of like the way iPhoto (and other mac apps) store their content kind of like a database file. I like it because to manipulate a photo in another program such as seashore you have to make a duplicate from iPhoto then manipulate the duplicate. Because of this, I find that I am less likely to somehow mess up the original photo.
This is the one thing I HATE about iPhoto/Aperture. Try uploading a file to a website using their basic browser-based uploader. Try to find that file by navigating to it like you think you would be able to. Nope. There's a database file there. And you can't "View Package Contents" from within the browser.
So, you are forced to make a copy just to conveniently upload, or to find the exact Filename for each and every photo you are uploading and manually search for it from the search bar in the browser window. You would think that something like Safari that is so "integrated" would be able to navigate through iPhoto/Aperture's database, but I guess not.
Back on subject, I have not yet used the new version of Picasa, but the old versions were nice, but iPhoto/Aperture still wins in my mind.