Same problem here. I used the Photos app in 10.3.3 to upload my large photo library (175GB, about 121,000 photos, 150 videos) to the iCloud Photo Library. These appear to have synced properly to the app across my personal and work laptops. After two weeks, my iCloud Photos web page still tells me it's "Preparing your library..." And with Optimize iPhone Storage selected, the Photos app on my iPhone running iOS 8.2 has been stuck at downloading 114,000 images for a week. This after wiping the entire phone once and reinstalling everything. I've got tens of thousands of white boxes in the app. When I select one, the progress wheel suggests it's downloading until it reaches 1/4, and then it stops. Wish I could download the iOS 8.3 beta to see if that addresses the issue.
Apple's willingness to lease us a terabyte of storage suggested a suitably large limit to the size of our media libraries. I'm hoping that will be the case, but if not, it would be helpful to know what the actual limit is. Any suggestions from Apple or others who've found solutions welcome!