1) Big names are using it
Prominent companies such as AOL and Warner Brothers Records has been using Ubercart for a while. More recently, Lullabot launched an Ubercart site for their conference registration.
2) Stable release for Drupal 5, significant progress on Drupal 6 version
Ubercart for Drupal 5 has been at a stable 1.x branch since the beginning of June. While it’s not yet stable, there is a dev version out for Drupal 6, and I know of at least one brave soul using it on a live site already. This is a testament to the active development community of Ubercart.
3) Ubercart is highly represented at Drupal events
There have been sessions on Ubercart at the last three Drupalcons, numerous local Drupal camps, FrOScon, etc. Ubercart even held their own Ubercamp this summer, for developers and interested parties to get together for a weekend of coding and planning.
4) Very active community
Dozens of new forum posts a day from its 3500 registered users, numerous code commits every week, and several community-contributed modules added or updated each week. The support time for issues and forum posts is very quick.
5) High visibility outside of the Drupalsphere
This is nearly unheard of for Drupal modules, but given Ubercart’s high visibility, it’s actually been a big draw for users coming into Drupal itself for the first time. They also recently had a mention in PC Magazine.
6) Full time developers with commercial backing
While the Drupal project itself has been proof that full time, funded developers aren’t needed to create a great product, in the world of e-commerce, it’s certainly helped. There are currently two (soon to be three) full time Ubercart developers, and an estimated dozen or so more from other organizations who also do only Ubercart development.
7) Focus on being user friendly
The Ubercart developers don’t want you to have to be a developer to set up an online store. There are a million settings and configuration possibilities for creating an online store, but they’ve put a strong focus on the UI, simple checkout, and intelligent defaults. This goes a long way in deploying a site right the first time, that looks great.