Of course it isn't. Theoretically, meaning on paper, yeah the philosophy sounds okay: use your opponent's momentum, using redirection to combat aggression, and all that stuff. But then again, Aikido emphasizes non-aggression, which equates to non-violence, and that does not make sense in a real fight.
But all Aikido I've seen fails for two reasons: in a fight, you cannot afford to be "soft", and they do not train in an alive manner. In a fight, even if you are being attacked, you cannot rely on the redirection of force. Not only is this rather difficult to do, because things can happen very quickly, but the fact that Aikido doesn't really have any attacks means that you cannot utilize a preemptive strike. Also realize that Aikidokas do not spar. They might think they do, but it is not in an "alive," or realistic, manner. This means that the techniques you do learn probably won't work because you've never used them on a person that is actually trying to hurt you, not somebody that's just standing still, letting you apply the technique.
All in all, no, I wouldn't trust my life to Aikido. It might work for some (not really), but for me, I'd rather study more credible systems.
If you're still interested in Aikido, just ask yourself two things: Why would you want to study a non-violent martial art; and if you don't think it looks effective, why would it be?
Warning: Randori in Aikido is less realistic than it apperas.