Aikido can be very effective, depending on the school from which you learn and your personal ability.
Aikido branched out shortly after O-Sensei Morehei Ueshiba (the founder of Aikido) passed away. Each branch, driven by different personalities emphasized different aspects of O-Sensei's teachings. Some emphasized the spiritual aspects. Some schools emphasized finding the exact angles and movements where your opponent is not being resisted, and cannot resist you. Another branch kept the martial power that was originally part of Aikido.
I am fairly new to Aikido, but have had some introduction to differing styles of Aikido through seminar training, and from contact with some black-belts that left another style of Aikido and joined ours.
Some Aikido schools (ryu) have become so focused on being light of foot, light-handed, and finding those perfect angles of non-resistance, they have become little more than fancy dancing (from what I have been told). Others retain some of the martial power. Many schools have dropped the weapons training that was originally part of Aikido (weapons techniques and empty hand techniques have parallels and connections).
My own senior instructor spent 9 years in Japan in Iwama Province (the birth place of Aikido) studying in the founders dojo under Morihiro Saito Sensei, who was given the founder's dojo when O-Sensei died. We study the weapons, and can generate power in our techniques to immobilize and opponent (at least my senior students and instructors can). I have personally used my little training to restrain teenagers who were getting too rough, and have used it successfully in Shotokan Karate free-sparring.
The downside of Aikido is it takes longer to learn than most arts. Many schools are more expensive than those of other martial arts, and it does not prepare you to grapple on the ground.
The effectiveness for an individual depends on their level of training, their personal commitment, how well they have stayed in practice, and their mastery of the psychology it takes to render an effective defense. This is true of any martial art, however.