I have used every forum software out there, free, or pay. I own licenses of vbulletin, threads, had classic before. You name it, I've used it. And as I've said before, nothing compares. Those free platforms that say "full scalable" are full of ****. They are so bloated with neat little features and bells and whistles that any large community (thousnads of members, and hundreds of thousands of posts) will simply not hold up to the task. Yes, they may still work, but you will be spending thousands of dollars on server upgrades just to make it run as fast as you want, which will cost you more than threads would have to begin with.
The other forums, from an admin standpoint are a nightmare to try and manage and customize. Some paid forums even, if you want a custom "skin" or new look for your site, be expected to shell out more money. Here with threads, you can get dozens of FREE stylesheets, or if you're friends with someone, they may even design you one for free or provide assistance.
And of course you have security issues... look at all the exploits available out there, but you're opening yourself up to being hacked, maybe even your whole server brought down. Yes, the free forums try to release the updates, but you'd think they would design their software more securely to begin with
With threads, I think there was one little security flaw mentioned as far as I can remember in the past few years, and that was promptly fixed.
So IMO, it's a small price to pay for a professional product that is going to hold up to a very rapid and growing membership of a popular forum. I also know I'm becoming part of a very active community and can get hacks, mods, or help with almost anything I need. I've had people, for free, write custom code for me. I mean that is just awesome. But you really can't go wrong, since when it comes to software, a lot of times the old cliche "You get what you pay for" holds very true.