I've bought one recently on Amazon.co.uk. I was around 45 pounds and comes with SD size adaptor, in which microSD card stucks a bit and it is tricky to take it out. I'll be using SD adaptor from my Samsung card, it is really easy to remove microSD from it.
Card is preformatted as exFat and tablet said that it is corrupted and needs to be reformatted, then stuck on unmounting operation. I'm on AOKP rom, btw.
I was expecting problems with file system and I've tried to reformat it to Fat32 on my PC (Win7 HP x 64) using few tools (HP format and Fat32formatter) but it didn't work either. I've also tried NTFS just in case if AOKP handles it but it didn't. I'm not sure if it depends or rom or kernel.
Then I've searched xda and found some threads with advice to use mobile phone (HTC Sensation or Samsung) to format card. Luckily I own HTC Sensation, so I've put card in mobile and formatted it. After that, card worked on my TF101.
Still it is Fat32 file system so it can't handle large files (over 4GB) but I don't worry about it.
PS. I forgot to mention that after format it comes as 59GB.