The 1TB PCI-e solid state drive that comes as an optional extra for the new 13-inch and 15-inch MacBook Pro with Retina display is crazy fast.
Here's a benchmark of a late-2012 MBP featuring a 256GB Mini-SATA 6 Gbit/s SSD:
And here is the early-2013 MBP featuring a 1TB PCIe 2.0 x2 5.0 GT/s SSD:
This is fast. Insanely fast. It blows SATA SSDs out of the water.
It's important to note that the 1TB drive is faster than the 512GB or smaller offering because it is a 4-channel PCI-e drive, whereas the smaller drives are only 2-channel.
Does everyone need this speed? Heck no! But if you're working with large files, or processing a lot of photographs or video, then the speed bump is more than welcome and add tremendously to overall productivity.