I'm guessing Mainland owners and fans out-number either Koaloha or Fluke/Flea here on UU. I don't own any of these fine ukes, but intend to purchase a Mainland at UWC2010 in June. I've not read one bad word about a Mainland, nor one complaint about their customer service.
Mainlands are more affordable than Koaloha, and that may explain part of the difference. The most often touted feature of the Flea/Fluke may be it's durability, and that's a fine attribute, but durability and the ability to stand up by itself don't necessarily offset the instrument's unconventional looks/materials.
Not a day goes by without a rave review of a newly received Mainland on these pages. But I'm just sayin'....