The English patch translates a few menu items, items in Miku's room (IMO kinda a useless feature), and some random tidbits of text, though there's still a fair bit of Japanese around the place.  Also can give romaji song titles and lyrics, if that's important to you.

You should generally be able to figure your way around though.
Here's the list of main menu items: http://projectdiva.wikispaces.com/Main+Menu