I upgraded from the iPad 3 64gb 4G to the iPad 4 64gb 4G. I upgraded mainly for the faster processor, WiFi Chip and the Lightning Connector.
The speed increase is definitely noticeable, apps launch faster, and games such as NFS Most Wanted run far better. On the iPad 3 when viewing the cars they stuttered when spinning them as if running at 15fps, the iPad 4 is almost fluid (actually racing was fine on both).
My broadband is 100Mbps so the faster WiFi is a huge increase in performance for me, the iPad 3 topped out at 39Mbps on 5Ghz, the 4 tops out at 87Mpbs on 5Ghz, the same speed as my iPhone 5.
As I own an iPhone 5 having an iPad that also had a Lighting Connector was much more convenient.
Also out of the box my iPad 4 was flawless, no dead pixels, dust, scratches, the tint is 100% even and there is no backlight bleeding at all
This is the first Apple iOS device that has been flawless out of the box in a long time.
Here is my list why. Apple's quality control is the worst of any company I ever seen, I have had 20+ phones over the last 10yrs such as Windows Mobile 5, 6 PDA phones and candy-bars, two Android phones and every one has been flawless except Apple's
iPad 1 - 1 replacement due to the screen randomly touching itself, the replacement is still flawless today (mum has this one).
iPad 2 - 8 replacement, 5 had major dust issues under the screen the other 3 had half pure white and half nicotine yellow.
The iPad 3 - 4 replacements, 3 having clusters of dead pixels and 1 with a very uneven tint. One of the replacements was flawless from the white box, but before I even left the bar, a massive cluster of dead pixels appeared after 5mins, the Genius was very surprised and replaced it again, that one is still flawless 8 months later (my grandparent now have this one after having the first iPad 1).
iPad 4 - Flawless
iPhone 3GS - Flawless
iPhone 4 - Flawless
iPhone 4S - Faulty earpiece that rattled during a call (sounded like a blown speaker) and had 5 LED spotlights beaming down from the top, replacement was flawless.
iPhone 5 - On my 3rd one, 1st one had a stuck red pixel, the 2nd had a big spec of dust that was very visible, 3rd replacement was flawless.