I usually don't buy apps on my iPhone, I just get the free apps (because I'm a cheap bastard).
But after downloading the free version of Angry Birds, I was so addicted I had to buy it. And it was worth it, I can't tell you how many times I ran my battery down while playing it. It's one of those puzzle like games which just mesmerises me.
In my honest opinion if you don't have it go to the App Store and buy it NOW!! or not just really up to you.