I have no feelings strongly either way. I don't hate them or love them. I will say that for the most part sidebars are a bit redundant with information you can find elsewhere on the forum easily.
It has to be done right IMO, because it can really mess with the flow of your layout. If the sidebar is a lot longer than the forum, it looks ugly. In reverse, if it's too short, it also looks bad. If done right it can complement the forum or it can make it look worse.