(I'm standing as I type this)
I tried a nice floor mat and for some reason found it made it harder to work, so your mileage may vary. I'm now mat-free but I wear comfortable shoes.
I also recommend doing a test run with boxes before your shell out for the full desk or attachment. I find I stand for 30-50% of the day on average, sometimes 100%, some times 0%.
Height of the desk makes a big difference - you want something that works for you both standing and sitting. I'm on the taller side and use the varidesk.com pro 36. My monitor is on a stand and still slightly shorter than ideal. My shorter colleagues fare better with the ergotron, which can adjust to a lower keyboard height when sitting.