Speaking from personal experience on Yosemite: slow UI, constant beachballs and stutters (despite lots of RAM and no platter drives), paint/refresh bugs while scrolling, 100% CPU usage to play tracks (without FX/EQ), years of mission-creep, and a general conceptual mess since version 12, with its multiplicity of modes and sub-modes.
As an iTunes apologist and power-user since 1.0 (my huge library admittedly is not helping), 12 has become the last straw on using iTunes as anything but a sync tool and database. As a media player, it's a disaster.
Good to know I'm not the only one. I've become used to scrubbing the pointer up and down over the list of podcasts until the part I'm looking at is legible, and plugging my iPod in twice to get it to show up. But IMO iTunes is actually one of the less-bad recent Apple software screw-ups. At least it doesn't have enormous memory leaks, like Safari and Keynote, which regularly beachball my poor laptop with "only" 4GB of RAM long enough to go make a cup of coffee.
For me that meant 30 GB of podcasts gone, most of which I can't download again (publisher went broke, original download required payments etc.) Sigh. I used to be a big fan of iTunes as well.
Podcasts used to be simple: follow an RSS feed, download the audio files, listen to them, delete them when you're done. Now they're "cloud," so your computer is just a cache, and God only knows when you have those files, or when some program lets you use them.
As an iTunes apologist and power-user since 1.0 (my huge library admittedly is not helping), 12 has become the last straw on using iTunes as anything but a sync tool and database. As a media player, it's a disaster.