I own a Hubba, and for the last year or so have been using it as my primary backpacking tent (except for an experiment with tarp and bivy, which in the end I just didn't like.)
The thing I like best about the Hubba is its versatility. You can pitch it without the fly on a clear night, and effectively sleep under the stars - without the bugs. In cool weather, when the bugs aren't out, you can save some weight and pitch it with fly and groundsheet only - effectively a tarp. The fly-only setup is a quick pitch to give you shelter from the rain. And, there's the full body-and-fly pitch for weather protection, with the vestibule closed or half-open-left or half-open-right depending on rain direction.
The ability to pack body and fly separately also means the wetter fly won't "contaminate" the drier body - giving you a little bit drier tent the next night. (You can't do this with a single wall tent - of course, I've read that the single wall tent dries very quickly once pitched.)
The Hubba is small - I sometimes find my shoulders brushing against the mesh walls. Fortunately, there's pretty good separation between the mesh and the fly, so you don't end up brushing the condensation-laden fly. (Condensation is a problem in both single wall and double wall tents when they're buttoned up tight; more so in some than others, depending on vents and amount of breeze.)
I like the Hubba, a lot. But, having said that, I'm very seriously flirting with buying a TarpTent Rainbow to replace - or at least augment - it. The Rainbow is a virtual single-wall clone of the Hubba, except that it isn't inherently free-standing. It's also wider and lighter, though the vestibule is smaller (the extra width offsets this very nicely.)
I don't agree with the other posters that the Hubba is "heavy" - but then, my frame of reference is my old Eureka Timberline with the twin vestibules - about 8 pounds, all told. Even my previous Sierra Designs tents, at 4+ and 5+ pounds, were heavier. So, to me, the Hubba is a lightweight tent - but I won't mind saving another 1-2 pounds if the Rainbow works out.