Hi guys,what forum do you use?
Is it a good idea to purchase a copy of Vbulletin?

All depends on what you need and want.
I have zero "admin-side" experience with commercial forums, and a lot of annoyances as a user... but latter part might be partly caused by lack of familiarity. Anyway, why not check out the well-known fre http://www.phpbb.com/ and http://simplemachines.org/ for start?
I'm personally a huge fan of SMF, it's customizable, works good, and doesn't have security holes overly often. Can't tell how it'll behave if you dump 20 000 users and half a hundred of badly written plugins in, tho.
Some people claim that for large forums phpBB3 works better([citation needed];)). I don't know, it always seemed "heavier" to me.

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rsk, providing useless advice on the Internet since 2005.

I use PunBB at a small fansite. I guess it has fairly limited features compared to the others mentioned above, but seems fast, and completely fulfills the needs of that site.

We use phpbb3 for a small forum (~100K posts).

Ditto for one a little bigger (~300k posts)

I've been using SMF and loving it. I can't remember exactly what I didn't like with phpBB, but I do remember finding SMF's scripts for porting phpBB forums into SMF. It went pretty seamlessly and I've been happy ever since.

I'd be interested in hearing from someone with a large forum to see if it has trouble handling more users.

IMHO, SMF is the best of the free forum software applications.

Invsion Power Board is currently the best commercial package, especially when you consider customer support.

vBulletin hasn't been the same since they were inhaled by a large US firm. They came out with a very buggy version 4. It still has a lot of bugs after just about a year in use. Some would also say that the new companies marketing leaves a lot to be desired.

XenForum is the new commer. It is being developed by the originators of vBulletin. It is still in beta but has a lot of promise and a large following of former vBulletin admins.

If you want the most polished system currently on the market go with Invision Power. If you want the best overall from the open source side, go with SMF.

Just my 2 cents worth, Jeff

I've been trying out JForum, seems easy to set up and use.

Drupal, since every site that needs forum functionality also needs to do a dozen other things.