I agree with -boRg- in his assessment that the Active Users function places extra stress on an Access database and increases the chance for a database corruption and also increases the database filesize more than if the feature is turned off.
That being said, I have had a forum running on Access with the Active Users function turned on for about 7 months now without a problem. There are just over 80 members, during the day there may be 15-20 concurrent users, and they have posted almost 50,000 posts so far.
I purge older posts from the database periodically, trying to keep the post count at about 10,000 posts, and I compact the database once every week or so. My host does a nightly backup, so if anything catastrophic happens I can restore if need be.