It's a delicate subject to me. In a way I sense weaknesses in Aikido against BJJ and maybe even some forms of kung-fu that use many fast low kicks, but I have faith that after years of training I will know how to react to them. Being in the early stages of my training I don't feel comfortable with the thought of facing anyone that knows BJJ well. It seems Aikido is really effective when your techniques are very good. Even O'Sensei said it wouldn't work unless done just right. Anyone can jump on somebody and do a crazy BJJ arm-lock with a little training....just a difference in the length of study needed?
The nice thing is, most people who study any martial arts aren't the ones that are out jumping people and starting fights. There's a mutual respect.
Hope that didn't come out too bad :P