The thing I like about the Customizer is how it brings standardization to the options being added by themes and plugins. It's not just the way it keeps a consistent UI but also the registration of fields and extending them, which will be very successful long term.

The things I don't like are mostly related to a feeling of confinement and restriction. I don't like the options being forced into such limited real estate and while that will probably evolve, it keeps me from wanting to use it. There are things that don't belong in this interface and I'm afraid the more it gets extended into other areas of WP the more we will see those things in the Customizer. I actually think including widgets in the Customizer is great. I almost like that more than the theme options in some ways but I don't love the existing widget interface and this seems like a much better one.

I'm trying to keep an open mind about the Customizer and for my own work I have built the options to exist in the WP admin with an option to replicate any of them in the Customizer. This way we only put relevant fields into the interface and keep them available to users that prefer working in the admin also.