If you are unhappy about the resolution of your iPad 2, it's definitely worth it. While I really liked how easy it was to switch between portrait and landscape when I first replaced my MacBook Pro with the iPad 2, the reality was that it was only useful with apps like Instapaper and Reeder which only show the content of an article and give you better formatting options. (I have a Kindle for books)
With websites, especially forums, text was usually too small/low resolution to be clearly legible in portrait.
With the new iPad, text is legible at virtually any size. The only time I have noticed that the resolution isn't quite enough, is with small Japanese characters and italic text in thin fonts. (I don't read Japanese, but I tried out a few websites to see how it looked)
I have no problem reading in portrait or landscape now, and text is clearer in portrait without zooming in, than it was on the previous iPad when you did.
If you had an iPad 2 and the screen
didn't bother you before, and you're happy to wait, I'd consider waiting for the iPad 4.
So far, every single app I have used, has run slower than the iPad 2. Sometimes it's just a slight difference, where an animation that used to be smooth will stutter now, but in some games (particularly those that just got retina updates) and apps, there is a significant drop in performance.
I was concerned about this when I heard that they were keeping the same processor and only doubling the graphical capabilities, when the device had four times as much to render, and unfortunately it seems I was right.
Even tasks like viewing PDFs in iBooks are much slower, with pages taking several seconds to render. (I know that doesn't sound like a lot, but it is)
The battery now takes a very long time to charge, and the device gets about as warm as the iPad 2 did when charging, through normal use.
It's noticeably heavier now, though it does feel better balanced, and the new shape feels better in the hand. Other than sitting the two next to each other on a table, I haven't noticed the extra thickness though.
In addition to the extra resolution, the screen is far superior now though. Colour is significantly improved, and it seems to be much more accurate overall. Here's a comparison posted on another website:
http://www.neogaf.com/forum/showpost.php?p=36093388&postcount=6130
That said, the viewing angle seems worse, it tends to shift towards a slight yellowish tone more than my iPad 2.
But I was frequently running out of space on my old iPad 2 (16GB WiFi) wanted the option of mobile data access, and really wanted a retina quality screen.
So I'm happy with the upgrade, but disappointed that it's the same speed or worse than the old model. I was already finding it to be painfully slow to render some websites on the iPad 2. (usually it's fine, but still slower than a laptop)
Hopefully with the next iPad we will see a speed boost, both to performance and charge times. 802.11ac wireless, and worldwide LTE support would be nice too.
If you still have an original iPad, it seems like its probably a must-have upgrade. Thinner, lighter, faster, improved in every way.
Oh, and I'm much happier with the black model than the white I had before. It just looks much sleeker when the screen, camera and microphone all blend with the bezel.