I've never known them to be useful, ever. Whilst you might get some new members, at the same time you risk any potential members following the affiliate and joining a site in the same niche that's better than yours.
Only one of mine has greatly piad off (and in fact his forum was bigger, he quit his forum and becaame the #1 poster at mine).
Point is that if you are going to do affiliates, make sure they are the same general area. If you are a PC forum, don't affiliate with a football forum.
Affiliate promotion works best when you are promoting a service that is of interest to your visitors / members, but that doesn't compete directly with your own site. For example, hosting affiliate schemes can work well on webmaster sites. Hosting affiliates also work well on sites that provide free hosting only. When a member is upgrading to paid hosting they will often use the affiliate link from their free host.