ComputerMinds have been working with Rex London (then dotcomgiftshop) since Drupal 5 when we needed to build a fully fledged eCommerce site. Back in the days of Drupal 5, Drupal was much less refined than its current incarnation so we had to undertake a reasonable amount of research to figure out how we could best meet our client's requirements.
Once the main development phase was completed the client wanted to continue to work with us on a rolling programme of support and development, including a full port of the site from Drupal 5 to Drupal 7.
An eCommerce Solution
As is reasonably common we skipped a major release of Drupal meaning that we jumped straight from Drupal 5 to Drupal 7. Due to the nature of how Drupal is supported only the current and immediate previous major release are supported at any given time. This meant that when Drupal 7 was released support was dropped for Drupal 5 so we had to start the process of porting over as soon as Drupal 7 and key contributed modules were stable.
Today when developing an eCommerce site our first port of call would usually be the Drupal Commerce suite of modules; however, when the time came to port over to Drupal 7 a stable release wasn't available so the decision was taken to continue to use Ubercart as this had served us well in the past.