I'm a die hard Drupal fanatic, but Drupal is more of a CMS and OSCommerce is more of a Online Shop application. Question is, should I stick with Drupul for my next online shop project, or dare to take on OSCommerce?
Tough to answer without knowing more. Drupal is not very strong if all you're doing is building [x], where [x] is an online store, blog, forum, rss aggregation site, etc. We recently retooled our company store in Drupal using the Ubercart plugin suite, though, and were able to exercise a lot of control over the final results -- and more importantly, intgrate it better with the rest of our site's content.
That's where the real win is -- if you have lots of existing content and/or community, and you want that integrated smoothly with your store. We can do things like auto-suggest products from the store that match the tags on the articles a user is reading, give people access to private forums on our main site based on purchases they make in the store, etc.
If you aren't already an old hand with Drupal, and you don't need that kind of connection, it's probably better to go with a dedicated solution.
(Random notes: Article about putting up the store, podcast about same)
Last autumn, i created several online-shops at a time using Ubercart. There were issues that were hard to solve (e.g. shaping the checkout-view), but in general, it was a good experience, mainly because you have all advantages Drupal has to offer, for example, products are nodes, and the way you present them can be tweaked using all the tons of existing Drupal modules.
I used OSCommerce once, and I nearly went mad adjusting the look as the customer wanted it. So my experience on OSCommerce is pretty bad, and i strongly reommend using Drupal.
If you're going to just create a plain store, without content integration, like Eaton suggests, you'd better go with a dedicated ecommerce solution.
This being said, if you decide to miss on the "extras" coming from using an integrated CMS/ecommerce solution like Drupal+eCommerce or Drupal+Übercart, you should probably not be using osCommerce anyway, but rather look at something with a different code base, like Magento or ZenCart (which derived from osCommerce originally).
If you chose the Magento route, specifically, you will find it is still possible to add Drupal in the mix afterwards if needed, thanks to an existing connector.
OSCommerce may be a practical solution but are you willing to maintain two code bases in the future? Your client will expect ongoing support and you'll need to be proficient in both Drupal and OSCommerce.
I would stick with Drupal if it does what you need. You'll spend less time fixing problems.