It's laggy, stuttery, junk. It's basically the only non-game application I can run that'll spike my CPU to 100%.

Big, slow, bloated, heavy-handed with settings (no, I don't want Safari, or QuickTime, or for you to phone home every five minutes), leaks memory, installs wads of silent 'helper' processes, non-native UI, frequent crashes, insanely jealous of all other ways to add songs to iPods...

It's very very slow, it crashes often, the UI is weird and hard to figure out (maybe it's more intuitive for Mac users?) and it has several annoying bugs: when syncing with my iPod it often marks podcasts I've listened to, or partially listened to as unlistened; it often hangs when trying to connect to the iTunes store (this is remedied by running "ipconfig /flushdns" at the command line); if I buy something from my wishlist it only removes from the wishlist about half the time. And every time it updates itself it tries to install Safari...

It is a sub-standard app based on a cross-platform layer of software (Carbon) that has not been updated to take advantage of platform enhancements (i.e., Windows 7).

Granted, it's not third-party since Apple owns the Carbon layer and iTunes, but then again, Flash isn't third-party for Adobe's in-house developers for the very same reason. Perhaps this is what Jobs means by "painful experience".

iTunes isn't built on Cocoa: http://daringfireball.net/2009/09/itunes_and_cocoa