PCI-e SSD user replaceable?

Well the rMBP uses a special SSD form factor. So yeah you can replace it, but nobody sells a compatible SSD as of right now.

 

Further

Watch for a new release in upcoming months by OWC for this PCIe. Be aware, however, that iFixit has ripped this appart and given it a 1/10 (one being the worst) grade in disassembly. Apple is doing their best to ensure their systems don't ghet opened, mutch more than simply using pentalobe screws now it seems.

 

iFixit's score has nothing about such an easy procedure as SSD replacement.
It's still a very simple process:
1. Unscrewing the back panel screws, taking it off
2. Unplugging the battery
3. Replacing the SSD.
4. Assembling it back.
Will take no more than 5 minutes.

The SSD itself is a way different story. Noone sells it separately at the moment, not to say about the external enclosures for it.

 

Looking at the ifixit.... It looks rather simple to replace the SSD. I don't think anyone makes them yet, that you can buy though. Someday, Im sure they will. I believe it will take a special bit to remove the screws on the cover. Other then that it looks like plug and play and its out in the open.