I had an Ipad 3 and just replaced it with an iPad Air. Both 32gig wifi. I'm 'food chaining' my ipad 3 to my oldest son (these kids are now into 'Clash of Clans' which is IOS only)
'naked' its noticeably lighter and smaller. It's definitely spunkier. This is the first time tablet web browsing feels almost as fast as a modern desktop.
I did buy the apple smartcover with the full back. I'm not sure I love it. In leather, its quite pricey I used to have the magnetic screen only smart cover with a clear plastic like wet apply wrap on the back of my ipad3. That back has kept the underlying aluminum scratch free.
The new smartcover, covers the beauty of the new back, and worse, definitely makes it feel bigger and heavier than if I just put a wet apply wrap on the back. It's also quite hard to remove it when you want to show off the naked unit. It really clamps on tight and you feel a nice pinch on your finger trying to pry it off. I'm not sure if they even sell the old style smart cover which is fine for a responsible adult, but not if kids are going to handle the thing.
IMHO, its worth upgrading from the ipad 3 or earlier if you have a good recycling use for the old one. I still get the nagging feeling they purposely didn't put in the finger print reader just to have something to sell to fanboys next year who bought an Air this year and want to buy next year's model. It would have been a nice improvement. Until curved and flexible displays become the norm, I'm not sure apple can do much more with the form factor of the 9"" model without sacrificing battery life so next year will probably be yet another speed bump + finger print reader type change and nothing more.
It's definitely the best tablet money can buy at this point but the Kindle Fire Hdx (which is even thinner and lighter) is probably the best value currently.