SanDisk 64 GB microSDHC (Ultra Class 10 UHS-I) works flawlessly with the Asus TF300T.
No setup was required. As soon as I plugged it in, the TF300T found the card and listed it's capacity (59.4GB) in settings. The file manager that comes with the TF300T was able to read and write to it immediately. Transfers within the TF300T to and from the card appear to be very quick. I haven't benchmarked it to test the advertised 30MB/s, but I am satisfied with the speed I am seeing. I would be very surprised if the TF300T was actually able to do this.
The TF300T does not appear to merge the flash drive into the file system as my Evo3D does with flash cards. This is rather unfortunate. My Evo3D will mount the card under /sdcard and will use it for program installation and support files. The TF300T appears to mount the card as if it were a USB drive and keeps it separate. I'm guessing I'll be able to fix this later through terminal on a USB connection.
The 128 GB for the keyboard still isn't in. It'll be in tomorrow, but I'll be out of town so I wont get to test it till next Tuesday.
My Evo3D actually doesn't like the 64 GB card. It comes up with an error. This could be a hardware limitation or a driver limitation as the Evo is running ~2.3 and the TF300T is running 4.0.3. I'll troubleshoot this more when I get back.
I'll post this in "The OFFICIAL "I got my Asus Transformer Pad 300" thread" as well. It is not my intention to spam the site with redundant information. My intent is to put the information where it might be looked for by those who are against using the search bar before asking a question. If this is unsatisfactory, please delete which ever one the moderator chooses.