I spent this Thursday and Friday migrating a pretty complicated site from 2 to 3, and boy it is a pain. My word of advice to anyone reading, do not use v3 for a desktop oriented website unless you specifically desire new hybrid grid system. It doesn't have fixed width elements anymore. It doesn't have truly flexible elements either. Form inputs are now screwed up (flexible) by default, lots of things are screwed up, some examples on the official website are still screwed up. It wasn't worth it, it looks and functions worse than it did, even with a bunch of new custom CSS.
