In my own opinion I would say it is. I had a 16gb 3GS and got the 32gb 4. The main reason I love the 4 so much is the camera, I absolutely hated the camera on the 3GS and hardly ever used it. I now find myself using the camera on the phone all the time now.
The screen is amazing!! I think that's all I need to say on that.
I also love the new design, it isn't without it's flaws (signal dropping if held at the bottom left) but it really is the best looking phone I've seen.
Saying that though, iOS4 gives the 3GS a lot of the features the 4 has, but for me, the screen the camera and the design make the buying one more than worth it for me.