As is often the case, I found this thread in google while looking for other information. I know it's an old thread but I just wanted to add my 2 cents about Aikido and Judo for police officers.
As to taking too long to learn, all I can say is it is like anything else - you get out of it what you put into it. If you practice regularly and diligently, you can progress quickly. If you want to be considered an "Aikido Master", then you are looking at decades of study. But that is a different goal than trying to become proficient enough to apply the techniques in self defense or as a police officer in the field.
Yoshinkan Aikido is more popular with police departments than other styles of Aikido. But I also know police officers who train with the Aikido Association of America and they swear by it. One Sheriff's Deputy who was teaching a class that I was in addressed the 'Aikido does not work" theory by saying "who ever said that never saw the look on a suspects face when he feels the locks".
To my knowledge, all Japanese Police Officers are required to be black belts in either Aikido or Judo. Many train in both.
Judo is excellent for police officers. But you have to train with a law enforcment oriented group. If you are a police officer training at the YMCA, a community center or a competition oriented club, then you will not get your needs met. But that can be said for any martial art. You have to train with someone who understands the needs of an officer and train with those needs in mind.
Take a look here for an article on a Judo seminar held for Police Officers: http://www.deputysheriff.org/INFORMANT-Winter04.pdf