They're also special because they fruit over a long season, unless a hard freeze blows in and ruins the fruit. I lost half of my Meiwa kumquats to the latest freezes. But the trees are fairly cold hardy, and supposedly able to withstand low temperatures into the teens when dormant. I didn't protect mine at all during the most recent winter blasts, and the lowest temperature at my house was 19.7 degrees. So far I haven't noticed any damage other than minor leaf burn.
I grow two kinds of kumquats right now, the Changshou and the Meiwa. The Meiwa is small, round, and relatively sweet, but the Changshou is by far the best in my opinion. Changshous are big, bell-shaped, only have a few seeds, and are packed with great flavor. Both are small, productive, well-behaved little trees. I have them planted in 30-gallon pots under my pine trees. Thankfully, the squirrels haven't been too interested in them, although I do occasionally find one burying acorns in the mulch.
JRN Nursery carries several different varieties. If I have the money, I would like to add Nagami and Marumi kumquats to my collection this spring. These are tart to sour kumquats that I'm told make excellent marmalade, and can be used as seed parents for hybridizing citrus.