If you have a "new iPad" (3rd-gen), you have no reason whatsoever to upgrade right now. I call the newly-released model an iPad 3.5 because it is a very minuscule update. But if you're the owner of an original iPad or an iPad 2, you should read on for some advice.
If you go to the Apple website right now, you will only see updated photos on the iPad webpage. The 4th-gen iPad has the same form factor, same Retina Display, same everything, except for a performance boost and the Lightning connector.
It comes with an Apple-designed A6X system-on-a-chip that is half a step ahead of the A6 CPU that comes with iPhone 5. And there's the Lightning port. This iPad is the rumored light-upgrade to the 3rd-gen iPad with a Lightning port.
Original iPad owners, upgrade
This is seriously not a bad time to upgrade from your original iPad. The 3rd iPad was a great upgrade for you and you didn't grab it. It was a good choice, because that iPad got discontinued quickly. Consider this Apple's second chance to make you upgrade. You get an even better iPad 3rd-gen for the same price.
There are two reasons: Firstly, the original iPad is nearly dead now. You don't have the latest iOS on it and a huge amount of apps are dropping support for the very outdated hardware on your device. Secondly, if you wait longer, you won't be able to sell off your tablet for much.
Go ahead and get the 4th-gen iPad or buy the iPad mini if you like it. It runs the same apps, but it's low-powered and lacks a Retina Display. Apps dropping support is a big deal.
iPad 2 owners and doubtful iPad 1 owners, read this
Upgrading from an iPad 2 is kind of a messy decision considering Apple's new release cycle.
I have an iPad 2. I wanted to upgrade to the fourth-gen iPad, which I expected to be a radically different device from the third-gen iPad. Not skipping a model did not make sense for me. However, it turns out that Apple made the third-gen iPad disappear completely and replaced it with the fourth-gen minor upgrade.
I am deciding not to upgrade.
However, some people might want to and they have good reason to. It's a big performance increase and the Retina Display is seriously awesome. However, hear out my reasons for not upgrading and you can decide for yourself if you want to upgrade. My decisions are not to be followed by everyone.
(My) Reasons not to upgrade
Apple released the minor upgrade in six months. To me, it seems like they wanted the "new iPad" to have the Lightning connector. They didn't want iPad 3 buyers to be angry for updating the tablet they bought and calling it the same. But they still seem to have angered a few people, shoving them into this little pocket and making the 3rd-gen iPad disappear from their store.
The thing is, iPad 2 is still on sale as the 'previous-generation' cheaper model. This leads me to think that they will still come out with a redesigned 'true' iPad 4 sometime in Q1 or Q2 of 2013.
If they had continued selling the 3rd-gen model alongside the 4th-gen one, I would be certain that they have shifted their iPad refresh cycle and the 5th-gen will come out a year from now. That's why I am still holding back in the hopes of a brand-new tablet.
Even if they don't come out with a 5th-gen model early next year, I will wait for the next update before upgrading.