I know of an SMF forum with about 16,000 memebers, it belongs to a friend. There are couple of other forum that a friend is running too.
I use it and am quite satisfied. The routine maintenance (applying upgrades and other modifications) is painless compared with other free boards. That alone may be a good reason to use it, but it has other appealing features too. Those features line up well with my needs.
Some here have suggested that the license terms are unpalatable, so I suggest you look that page up and see for yourself if you can live with it.
i have recently switched to SMF from phpbb and I wish I'd done it so much earlier. In my opinion SMF wipes the floor with phpbb, it's simple to use, has ueful admin features and is quick too.
I have about 250 members on my forum and not one complained when I switched from phpbb. That's got to say something.
It gets my recommendation anyway
We use SMF and like it a lot better than PHPbb.
Ours is not that large, we get probably 300 messages a week in about 8 forums.
And I love the upgrade process - just click a button and it downloads and installs automatically.
I use it, we have close to 3,000 members and over 200,000 posts. I highly recommend it. I'm a member of another SMF forum that has double that membership and over 1,000,000 posts and they have never had any problems with it.
It's feature rich, is SEO friendly, has a great admin section, a great banning/post moderation feature, is quick, fully customisable and the support via Lewis Media and their support forums is great.
Edit: We switched from PHPB too and no-one complained about the forum itself, just a few complaints about the choice of skin/theme we adopted (custom designed), user wise, they all loved it.
There are many different 'designs' of it as well that includes a sidebard should you want to put ads down the side and such, or you can near enough rewrite the entire code if you know what you're doing. It meets W3C standards and can even be accessed via mobile technology. The best feature? Comes with a spell checker :)
I also have switched to simple machines from phpbb, and love it. I wish I had known about it sooner.
I just wanted to jump in and say that I'm running five forums with SMF after switching from phpbb and haven't looked back. Awesome software.
I'm also very happy with it - I run it on two sites and it has worked well so far.
I got SMF as I was fed up with phpbb.
It works wonderfully.
For big boards running SMF see:
You have to bear in mind that there are less SMF sites than other forums as it has only been running for about a year ;)