A brief statement and then I'll expand with some more thoughts:
iTunes is the sole reason why I started to even look at a 'Mac' back in the day. iTunes spawned OS X, OS X spawned UNIX, and UNIX ultimately gave me my career. So if I've said this once, I'll say it 1,000 times... I will always be indebted to iTunes. Then there's the whole love of music, my half-audiophile nature of my brain, and the massive vault that is iTunes' media library, (stripping all other functions from it, sticking with the media library portion of it).
That said, I've been eagerly waiting for iTunes 11, as iTunes "pre-11" had become outdated, bloated with features outside of 'tunes', and well, slow. The biggest issues I've had with iTunes pre-11 was how much of system resources it consumed daily. Since iTunes is a staple in my household (streaming media and all the other jazz), it's always open, and ALWAYS consuming at least 1.5 - 2GB of RAM.
Since I've had iTunes 11, I have used ALL it's features religiously, for a solid 24 hours now. It has only consumed < 650MB of RAM since I've opened it.
It's fast, it's slick, and the interface is radically different (not bad) as it's both logical and refreshing. It's great. Period. There's a little place in my heart that's so happy to see "my" software mature, finally.
I have only two modest gripes:
1. You can't change the size of the album art in the default 'Album' view (formerly known as 'Grid' view).
2. While playing an album in the 'Album' view, you wouldn't know if you've closed the new song list view. In other words, unless the album your playing is open, you can't tell what album is playing.
Other than that, it has my recommendation. Heres' a screenshot of my library:
Oh, and one more thing - one big change that I neglected to mention... music videos. They're separated from the album and properly given their own section. Yes!
The biggest benefit of all that is for those AppleTV users out there... it now has a 'music video' section! Freaking awesome... so go enjoy!