[Asus  Eee Pad Transformer Prime] Xerxes-3.0:
Okay, first, long story short to get an overview:

Stock ROM

Followed standard procedure to unlock and root

Installed (fastboot) TWRP 2.6.0.0 and PAC ROM 4.3

THEN did the NVFLASH procedure and recovered blobs

Installed modified CWM from startpost

Installed CM11 nightly

Brick

Recovered using NVFLASH, back to stock

tried again, "signature error" "unrecoverable bootloader error"

Now (Could NVFLASH back to stock, though)

Maybe get familiar with the APX recovery manual that vanquiche posted, I will mention it twice.

What happened:

1.) Stock ROM, updated to latest official (I think 10.4.2.18, Android 4.1.1)

2.) Unlocked Bootloader using the .apk from the ASUS homepage and generally sticked to the Cyanogenmod manual. The "Device is unlocked"-info on boot splash screen appeared afterwards.

3.) Downloaded/installed TWRP 2.6 for TF201 as described on the TWRP homepage. I think this is still standard so I will just link it here. I used PAC ROM 4.3 (well...it's on XDA) for a while and adb-pushed it, then installed from TWRP. Did several nightly-updates also, and had no problems with it. Not doing NVFLASH first may have been stupid in retrospective, but it is like it is now.

4.) NVFLASH access via this thread and AndroidRoot and got my files, I think this still counts as standard procedure. I had not really an idea on what I was doing and followed the manuals in detail. However I did not get the NVFLASH backup files from stock roml, but TWRP & PAC ROM.

5.) Wanted to change and installed namidairo's modified CWM from the download link in this thread. Actually I think I have not tried booting to Android afterwards, but I cannot remember.

6.) ADB-Pushed the unofficial CM11 from Feb. 20th from startpost and flashed it from recovery.

7.) Brick, Recovery not accessible, "Device is unlocked" did not appear on splash screen. APX worked. Wheeee....ly.

8.) APX-Recovery manuals ar scarce if you want to get beyond "You need it or you're screwed!". Looked for halp in this thread and after a short amount of time, vanquiche posted his recovery manual that he got from lj50036 from the CMB team. However, this manual, thich essentially said (do wheelie --blob blob.bin after every reboot because only APX was available)

Aferwards I should have been able to fastboot but it did not work. So I did some itelligent guessing and did

Then, I could do fastboot but stil no recovery ("unrecoverable bootloader error"). Knowing I already tinkered with the Prime before the NVFLASH backup, I turned to the thread [GUIDE] nvFlash General Users Guide, downloaded the UN-BLOB'D STOCK JB 10.4.2.15 ROM from startpost and extracted the blob files. Then I just guessed.

Again trying to install CM11 ends in abrick (again). This time, I repeated the recovery program from vanquiche's post starting at step 8. I installed the stock-ROM-blob WW_epad-user-10.4.2.18 from ASUS homepage and everything worked.

9.) Tried again, flashing mod. CWM or Voyager2k's 4.4-compatible TWRP 2.6.3.0 both leads to being able the stock ROM, but CM11-Unofficial ends up at saying

Signature mismatch
unrecoverable bootloader error 0x0...08

I can get back to recovery (no android) with

10.) Now.

11.) For the record, I did the APX-NVFLASH-Recovery again and went back to stock (again), installed Voyager2k's TWRP 2.6.3.0 (again) and installed AndroWook 2.31 with its own bootloader and recovery. It. works.