The Aikido community seems to be very caught up about Seagal Sensei, but I often see a lot of speculation and judgement without getting into the verifiable facts. If you want to evaluate Seagal, maybe a good place to start would be to look at the quality of some of his top students like Haruo Matsuoka, Larry Reynosa and Luis Santos.
Seagal's Tenshin Aikido is Aikikai, not Daito Ryu or some bastardized Aikido mixed with Karate. Years before I started studying Aikido, I was able to recognize and name the techniques he performed in his movies as classical Aikido techniques based only on descriptions of Aikido I had read in books. But, as has been stated before, he can't be judged as a Shihan under Hombu dojo by his movies, any more than a top Aikidoka could, for example, be judged by the quality of the products in another Aikido-related business he or she operated.
Seagal isn't the only teacher to focus his teaching on the martial effictiveness of Aikido. Yet he seems to get much more criticism for this than do Aikidoka who go to the other extreme, and focus heavily on ki almost to the exclusion of the martial.