If you don't want to be an unhappy Happy Pets player, you should read this post for a heads-up of what to expect from the game's new Mystery Boxes. Shortly after the boxes made their debut last Friday, upset players were already complaining on the official Happy Pets forum.
Here's a recap of the major gripes: 1) Inclusion of items that cost LESS than the boxes. 2) A seemingly ineffectual "Add Luck" feature. 3) No special items have been declared for the Mystery Boxes, meaning there's nothing new.