I've only owned one iPhone before which was the 1st generation one or also known as iPhone 2G.
And thus my review will be compare to that phone, which gives the iPhone 4 a massive advantage in this review.
First the interface, which has greatly improved since iPhone 2G, it is way more "snappier" and responsive. The processor A4 has improved that even compared to the iPhone 3G S. (So I have heard).
As for the display there is no question, the Retina display is currently the best out there, might not have the highest resolution, but is way more detailed, sharper and colorful.
The new design is all about your own personal preferences, some prefer the rounder phones and others the square. I myself prefer the rounder ones more, but the iPhone 4 looks absolutely gorgeous now with it's class front. (and back).
To bad the design has a drawback, which is the signal bars when you hold the phone in a specific way, (this specific way is actually how everyone holds the phone), but I personally have never experienced this problem, but that doesn't mean I don't believe it is true.
As for sound quality, the call quality has slightly improved from the iPhone 3G S, which I heard from my classmate, and of course greatly improved from the iPhone 2G. Speakers are still very bad, but as for ear-phones, I personally tested these with my Ultimate Ears TripleFi 10vi and it is quite good, /without a portable amp.
Internet has improved with the support of n capability, and it is noticeably faster
So there is my subjective view on this product.