I have had the premium edition since 3 weeks after the game came out. I’ve beaten the game 2 or 3 times, done several endurances, etc. etc. Have I ever once regretted buying it? Not at all. If anything, I’ve wondered how so much more content in a game was so cheap.
It certainly is worth the 5$. Personally.. I play GemCraft when I’m bored or just piddling around at work. If you do enjoy the game, this is a must buy, but if it’s just kind of so-so for you, you might want to move onto another game.