For those unfamiliar with the history of Aikido, here is a very brief summary. Ueshiba Morihei began studying Daito-Ryu under Takeda Sokaku in 1915, and his official affiliation lasted until 1937. In 1919, Ueshiba moved to Ayabe and began his study of the Omoto Kyo religion. There, he also began teaching martial arts unofficially. In 1922, Takeda visited Ueshiba and lived with him for six months, at the end of which he awarded Ueshiba a kyoju dairi (representative instructor) certificate. Interestingly enough, on Ueshiba's certificate is the first written mention of Daito-Ryu Aiki Jujutsu instead of simply Daito-Ryu Jujutsu. There are several theories as to why Takeda made this distinction, but my personal opinion is that while Ueshiba's teachings were still Daito-Ryu in nature, Takeda felt they had been modified too much to be pure Daito-Ryu Jujutsu. Throughout the late 1920s and 1930s, Ueshiba continued to modify techniques emphasizing the concept of mutual harmony - that while you can and should defend yourself, there is no need to cause permanent damage or worse and both parties should walk away. The continual evolution of Ueshiba's art is evidenced by the various schools of Aikido, ranging from the rigid and combative Yoshinkan school to the moving meditation of the Ki Society, and is due largely to the time frame that the respective branch founders studied with Ueshiba. Throughout the years, Ueshiba called his teachings Aiki Budo before settling on the name Aikido.
In Aikido, you are supposed to blend your movement with that of the attacker and guide them into the specific lock or throw you are attempting. Your body should remain in a relaxed state, the fluid movement should alleviate any resistance offered by the attacker and circular motion will compensate for the lack of power put forth into the technique by the tori/nage. In reference to joint manipulation, anytime you compress, hyper extend or twist a joint, you will get a favorable bodily reaction or distortion, and the principles previously described will create enough kuzushi (off balancing) to throw an attacker. So by looking at these principles, the answer is yes, Aikido can be an effective martial art.
That being said, principle alone will not protect you. You must practice under duress. You must train against live, realistic attacks. Shomenuchi and Yokomenuchi, as well as wrist grabs, are great for teaching the initial movements of a given technique however once you have ingrained the movements, you must train against more modern attacks (boxing-style punches, etc.). If your instructor is not exposing you to this type of training, regardless of what style you study, it's ineffective. The answer to Aikido's efficiency will be determined by the branch of Aikido one studies, as well as the instructor they choose to study with in that specific branch. It simply depends on the emphasis of the instructor. Are they teaching Aikido as mainly a moving mediation, is the purpose spiritual development or is the instructor pushing freestyle randori? Are you encouraged as an attacker to offer resistance or simply go with the technique at all times?
Neither approach to Aikido is wrong, and ultimately the choice is yours. Aikido can be a dynamic self defense art with powerful throws, devastating locks and even striking. Do your research, find the school that meets your training goals and expectations, and when in doubt, find an Aiki Jujutsu school (light-hearted, shameless plug)!