That is a matter of opinion. Roblox is a popular game with millions of users, but you will have to decide for yourself if it is fun.
Here's a little about Roblox.
Roblox is a MMOG where players can create virtual worlds with blocks of various shapes, sizes, and materials. Players have the ability to customize their virtual character with hats and clothing and communicate with one another through groups, built-in instant messaging, in-game chatting, "foruming," and private messaging.