The new Rebel is a minor improvement on the old Rebel. But is there a good reason to buy a new Rebel at all?
Let's just make this short and sweet. Canon has two Rebel DSLRs in their current lineup: The new T4i, and last year's T3i. Here's why you would choose one over the other, or choose a Rebel DSLR at all.
The T4i is Better at Video
The biggest tweak to the classic Rebel formula is the new "hybrid" autofocus system. It unlocks full-time autofocus in video mode, making the T4i first Canon DSLR at any price with that feature. And with stepping motor (STM) lenses, AF is smooth, silent, and somewhat reliable with moving targets.
Otherwise, it shoots solid 1080p video like the T3i and T2i before it. But it's still amateur video; a videographer would almost certainly not use the T4i. It shoots slightly smoother home videos than it did before, nothing more.
Canon also added a touchscreen to the T4i, which works well but is in our opinion largely unnecessary on a camera. We measured marginally better image quality than the T3i, but that probably had more to do with the new 18-135mm STM lens and DIGIC 5 processor than any changes to the sensor. Live view autofocus is improved as well, though it's still slower than the contrast-detect AF systems on the best mirrorless system cameras.
The T3i is Perfectly Fine, and Cheaper
With a largely unchanged body, virtually equal image quality, and a discounted price, the T3i is the better buy in almost every case. If you never shoot video, it's a no-brainer. Even if you plan to shoot the occasional video clip, the T3i is totally capable of capturing solid home movies. And if you don't have to have the touchscreen, you won't notice the difference. If you have your heart set on a Rebel DSLR, save a few bucks and nab the T3i.
You Should Really Look Outside the Rebel Line
Canon Rebels are reliably solid consumer DSLRs. That would've been a meaningful compliment a few years ago, but now it's faint praise. The arbitrary divide between "casual" compacts and big "serious" cameras just doesn't exist anymore. If you want something better than your cheap point-and-shoot, you have options besides a bulky, possibly intimidating Canon, Nikon, or Pentax DSLR.
Entry-level, interchangeable-lens mirrorless system cameras like the Sony NEX-F3, Panasonic GF5, and Olympus PEN E-PM2 all cost less than the T3i. They're also much smaller, and are laid out like point-and-shoots so that casual users don't need to fuss about with extra settings and buttons. Image quality is generally in the same ballpark and occasionally rivals the best consumer DSLRs. You might lose some luxuries, like an optical viewfinder, but LCDs and electronic viewfinders are improving by leaps and bounds with every generation.
And while we're at it, let's take the argument another step further: A huge majority of system-camera owners never even buy a second lens for their cameras. Why not buy a capable fixed-lens compact, like the Sony RX100? It's one of our favorite cameras and hey, it costs $650, just like the T3i.
The Canon Rebel T3i and T4i are both solid cameras, with the high level of refinement you'd expect from a company that's been doing the same thing over and over and over again. But there are way more players in the game than there used to be, so it'd do you some good to look around at the competition.