If Drupal can support the requirements of your project with minimal changes, absolutely go for it. Otherwise, stay away. Drupal can in theory be used to build a wide variety of full fledged applications, but my experience says that this is the wrong hammer for most nails.

I faced the same choice a few years back, in Drupal 5 days. Ended up going with Drupal for speed of development, would not have made the same choice if I had to do it all over again. Built a fully featured social network by stitching together a bunch of modules and writing a hell of a lot glue code as well as code to override default module functionalities.

In the end, I did build it faster than I would have from scratch, but ended up with an unmaintainable mess of custom code. Ramp up was brutal (I knew nothing about Drupal when I started) and you really need to understand how the core and module architecture works in order to make any meaningful customizations. The prospect of upgrading to Drupal 6 was so daunting that I never did it and the project ended up languishing and eventually dying.

Perhaps things got better since then, but I have a hard imagining that to be case.