The forum module of drupal is very basic and when compared
to other forums softwares like vbulletin or simple machines it lacks features
& functionality .

Question is why would you use drupal forums.? I would say
that if you want a basic mechanism for discussion and don’t anticipate a huge
group of users, then you can go with inbuild forum module as it fast and easy
to maintain. If you install a third party module like vbDrupal or Simple
machines integration then may have to wait for upgrades if you are not a
developer. Also you are at the mercy of
the free support offered by the community if you have any issues with the
modules.

First let me show you some huge sites using the Drupal Forum
module for inspiration

As you can see with some minimum theming you can achieve
wonders with Drupal forums. It’s fast efficient and easy to maintain.

Let’s look at some contributed
modules that will let you enhance
drupal forums.

1. Advanced Forum : Most popular module loaded with features you
   need. It enhances Drupal’s core forum module. The included styles provide the
   look typically found in stand alone forum software and the code provides some
   of the functionality, mostly related to the end user experience. By adding other Drupal contributed modules, many of which are automatically used by Advanced Forum, you can achieve much of what stand alone software provides.
   Because it uses the core forum module, it uses the node and comment system built into Drupal and is completely integrated, not a bridge.
2. Forum Access : This module changes your forum administration page to allow you to
   set forums private. You can control what user roles can view, edit, delete, and post to each forum. You can also give each forum a list of users who have administrative access on that forum (AKA moderators)
3. Signatures for Forums: Signatures for Forums provides user signatures that will be familiar to users of popular
   forum software.
4. Forum
   Thread: An implementation of a
   threaded forum view where pages show forum topic and comment titles in a threaded view with many forum topics on a single page. The module also adds two blocks that offer threaded display of the new and active forum topics.

Since drupal forums are essentially nodes, you can use any
other contributed modules to enhance your drupal forum site. For eg. You can use subscriptions module to enable users to get emails to the subscribed posts, you can use modules which enable you to have social bookmarking buttons for each forum post.

Migrating from other forums to Drupal.

1. You can now migrate from PHPBB forums to Drupal forums using phpbb2Drupal contributed module.
2. To migrate from vbulletin forums to Drupal forums use vbtodrupal contributed module

Integrating popular forums scripts with Drupal.

Drupal community has come long way and now you
can integrate all three popular forum scripts with drupal. Which means that you
can use best of both world. If you feel vbulletin has best forums and Drupal as best CMS simply use
module to integrate the two.
You can even integrate simple machines forums
with drupal
And finally for those running phpbb forums ,
here is how you can enhance you forum website by integrating with Drupal.