We just did (from SP21_G1)...
If you are using Solution Explorer, make sure all users are assigned default roles, or they will not see their menus.
Also important, check out p36-37 of the Installation Instructions as it relates to database password encryption. Depending on where you are starting, you may need to install an ESU in addtion to the Service Pack, or else your administration will not be able to change passwords/user status. Other than that, no big issues (knock on wood)...