It tends to put people off visiting and gives you a bad reputation... That means no one wants to join and the best method of advertising which is word of mouth will stop.
I wouldn't utilize PM advertisement. Instead, I would become a part of the "similar" community. After the community starts to trust you (which shouldn't take long if you provide helpful comments, don't spam, etc.), you can ask the Administrator if HE/SHE would let the community know about your site.
I think if you use this approach, it'll appear less spammy, and a recommendation directly from the Administrator, rather than you, will likely carry more weight. Of course, not all site admins will do this, but it does not hurt to ask.
PMing the owner works 10$ of 100%, but just posting on the forum is better, you get ur site there for google to index and even better that it will give u a better Page Rank.