Yes, I found that Safari was sometimes very laggy on my iPad 3. On the iPad 4, for the first time, browsing the web is as fast as on my 2011 MacBook Air.
Also, I often work with big PDF files. I've always preferred the "PDF expert" GUI to the "GoodReader" one, but "PDF expert" sometimes was very slow and it could crash. So I only used it with "light" PDFs. With the iPad 4 it's so fast that I can work comfortably with all my PDFs. And it doesn't crash anymore.
One more thing: when the iPad 3 was working hard (like when editing big and complex documents) it didn't only lag: it became very warm and the battery was being used even when in charge (albeit very slowly, but if I was at 1-2% the risk was to make it shutdown). This doesn't happen anymore: thanks to the A6x processor it goes idle faster, so it doesn't get so warm (yet it's still a little bit too warm for my taste). More importantly, with the new 12w charger the battery is never discharging when the iPad is plugged. And it's also alittle bit faster to charge. So, to me, it's a pretty big difference...add to that facetime HD, faster wifi, and a new and better connector.
I Only wish it was a little lighter. 500g would be perfect (ipad 3/4 are ~650g right now, ~600 the iPad 2). I've been very critical with Apple lately, but I'm really enjoying my iPad 4.