All-in-all, Lips: Number One Hits feels like a stopgap. The track list is underwhelming and there aren't enough improvements to warrant the full retail sticker price. However, Lips is still a fun game in concept and the next iteration will, hopefully, incorporate more gameplay improvements and have a better track list.
If you're looking to get into Lips, pick up the original. If you're already into the game, seriously consider taking the money you would have spent on Number One Hits and investing in some songs from the massive catalogue of DLC and waiting for the next Lips game to drop.