I feel that it depends greatly on what sort of attack you are defending against, and what circumstances you are likely to find yourself. A lot of people in my class are bouncers, and they use the techniques on an almost daily basis. In this case, Aikido is an amazing art simply because of the ability to control the other person without injuring them.
For the same reason, we also have a lot of cops, one of whom is a trainer for the academy. Again, this is great for people who know that they must take the initiative, are typically alert, and would be legally liable for causing more injury than was necessary.
For a stand up fight, you have a bit of a different story. Against linear arts, like karate for instance, it has an advantage of being circular in nature - and linear arts typically don't train for that. You also have an advantage in that most Aikido schools practice the rondori - multiple attackers - that helps in that sort of situation.
Still, there are two types of attackers that I have trouble with using straight Aikido - the boxer, and the soldier. The soldier is a no brainer here, if you are against an opponent that wants to kill you, you have to be in that same mindset to win. My sensei taught Aikido to American soldiers during the Korean war, but admits that he had to modify it to to a more brutal (and simpler) form for it to be the most effective in the given situation and the limited training time.
Boxers are difficult because of the jab. The job doesn't use the whole body, and is difficult for the Aikidoka to properly off-balance and lead the opponent. In my style, we incorporate trapping to offset this - but I have to admit it's the hardest thing I've tried to do. Usually, when I spar against boxer types, I stay out of range until they get pissed off and finally give me a good hit to work with - I need to make my traps more mindless.
This is the really difficult thing about Aikido, and all martial arts really. Knowing intellectually what you need to do to counter an attack is relatively easy. Training your body to execute the most appropriate technique without thought, while maintaining composure and awareness of the environment, is the hard part. This is why I think rondori is so important for the practice of Aikido, it forces you to stop thinking and start doing what you can to avoid getting clobbered.
One thing that may seem odd at first, you will never see white belts in Aikido doing anything that resembles combat. Hell, you really don't start seeing anything that looks like combat until you see two high Kyu grades or even Dan ranks working together. This is simply due to the fact that lower ranks can't fall really hard. Since they can't fall hard, they typically work out at 1/4 speed - different from the usual 3/4 speed for sparring in other styles. As you get better at falling though, the more speed and power you can use, and the more fun you have!