The S4 was considered a beast when it was unveiled several months before the HTC One series and Galaxy S3 emerged. The only complaint was a slightly under-powered GPU. I think it's the exact same with the Snapdragon 600 with Adreno 320. It will be great, but it will quickly lose appeal as Apple's new SoC and the Snapdragon 800 are in products only a few months later. Unfortunately, even the Adreno 330 GPU (which is a 50% graphics performance increase over the Adreno 320 in the Snapdragon 600) is said to fall short of the already old Apple A6X from last year.
That being said, the S4 in my Galaxy S3 is still quite fast and I'm very happy with it even today. Upgrading to the Snapdragon 600 is said to be a 40% increase over the S4 so that's already amazing...and I am definitely a believer in real world performance > specs on paper. Unfortunately, the tech world loves specs on paper...and I would rather wait a few months extra to get a HTC One or Galaxy SIV with a Snapdragon 800!