Profile backlinks are one major part of my backlinking arsenal. As you probably already know, backlinks are, beside proper keyword research, the most crucial factor of success for any SEO or internet marketing project.
There are a lot of methods, theories and more or less reliable case studies on how to create and manage your backlinking for a given SEO campaign. Profile backlinks, which by the way are not strictly limited to forum platforms only, become popular back in 2009 when there was the famous Paul & Angela backlinking service being discussed in any IM or SEO forum you can imagine. The guys were selling way overpriced packets on a monthly based, with links to forums where their customers could create profiles and drop a link. They did it successful.
And, in fact, backlinks from forums worked very well during 2009 and also 2010. The main factor is certainly the IP/domain diversity you will have in your backlink portfolio.
Recently, many experts and wannabe-experts are debating about Google devaluing inbound links from profile signatures and profiles. While the supporters claim that Google is not in the position to do it, as first you would also cut legitimate links and secondly, SEOs using a tool like Scrapebox could blast their competitors in order to get them de-indexed, the objectors tend to say that they observed negative SERP movements in the recent past for websites which ranked basically using profile backlinks.
Whether this is true or not, I cannot comment. What I can say is, that I still have pages ranking, which do have profiles in their backlink portfolio. But as I never limit any SEO campaign to one single method and type of links, I cannot speak for profile-only strategies. I strongly advise to keep an eye on diversity, not only for domain and IP diversity but also in concerns of your methods and efforts you put in a project. Never put your eggs in only one basket, is a good old saying. It holds true for SEO as well.
Another point you cannot just ignore when talking about profile backlinks is the indexing rate. As a matter of facts, links which are not indexed by Google are like non existing links. Another fact no one can deny, is that getting and keeping profiles indexed is getting more and more difficult. Google is catching up on this, and this is for a good reason. Why would you keep a page in an index which holds basically no information beside a username and a bunch of links? But there are a few things you can do in order to maintain an acceptable indexing rate for your profile.
First, build backlinks to your backlinks. This is true in general and for any kind of backlinks. A convenient, fast and easy solution is to blast your profiles with blog comments, e.g. using a tool of the like of Scrapebox. Furthermore, build your links over time. Just don't register 3000 profiles, put your links in them, and leave them alone. You will find yourself wasting your time. Better is to register a hundred profiles, maintain the about me and other sections profiles typically have, then come back a week later and throw your link in there. You would be surprised about the much better success rate. Last but not least, log in to your profiles regularly. This will make your profile appear on the main page of the forum ("currently logged in users" or similar), which will get it exposed more to the search engines. I hope you know that this is favorable.