I have to agree that aikido is effective depending on how you define effectiveness.
Effectiveness to me is not about being an ultimate fighting machine. I have a wing chun friend who is always pointing out the weaknesses in other arts, including aikido, and therefore, he knows how to defeat them and therefore wing chun is better. The the thing is he, admittedly likes to fight, and he wants to be a great fighter. I tell him to stick with wing chun then.
Effectiveness is about attaining the goals we wish to acheive through aikido. For me I have two goals self-betterment and self-defense. To learn these I need the practice of aikido. After only a year and half of practice, I can already see changes in myself. As for the self-better side of aikido, yes, it is effective. As for the self-defense side, I have yet to be in a situation where I was required to use it so I really don't know.
So yes, effective is relative to what we want out of it.