The most obvious benefit from the start will be lack of ads before/during videos. Later, they plan to start having premium content that is only available to paying subscribers. It's not clear from what I've read exactly what this content will consist of.
One thing that I don't like is that YouTube is forcing major content creators to sign up and participate in the program or have their content removed. This won't impact the average user, but big personalities will have little choice but to participate