Some people have complained that writing a snapshot on the DivMMC takes as long as a full minute. This is due to the cards used and because esxDOS at the moment does not do multi-sector block writes. If you use a card that has a smaller internal flash block size you can expect good performance (even with the current version of esxDOS). Here's a couple of good cards for reference:

Toshiba SD-R04GJ(BL5 [SDHC 4GB]
Panasonic RP-SDU04G [SDHC 4GB]
Sandisk Extreme [SDHC 8GB]