I am more of a tipi gal myself, but I have spent a few nights in floorless shelters, both golite and Kifaru tipis as well as the Kifaru Megatarp. I have never noticed any issues with rain coming into the shelters at all, but if it does it would be an inch or two into the interior at the most. I suppose the type of terrain might make some difference (we most often camp on tundra), but probably not much. With our SL3 and SL5 we usually leave around 3-4" of space at the bottom (and even inadvertantly a little more if the surface is uneven), and that little bit of airflow seems to keep the condensation inside the shelter to a minimum. If its really windy we will pitch it closer to the ground, but most drafts can be effectively blocked with gear or packs. I was skeptical at first, but breezes have pretty much been a non issue.
The Kifaru tipis and the mega tarp seem designed to stake out more flush with the ground, as the stake loops aren't adjustable (like the ones on the golite tipis are). We haven't had any trouble with condensation in our K tipi ( that might be due to size, it's a 12 man) and the open end of the Megatarp allows for ample ventilation if you aren't using it with an annex.
As an aside, I was concerned the Megatarp might be too breezy with the one side opn, but in three nights bivying in it on my sheep hunt this proved not to be an issue at all. I probably wouldn't want to be waiting out a gale on Kodiak in an open ended Megatarp, but for most other summer and early fall applications here in AK it is a great lightweight shelter.