He has written 18 books which combined have sold over 26 million copies
Three of his books, Rich Dad Poor Dad, Rich Dad's CASHFLOW Quadrant, and Rich Dad's Guide to Investing, have been on the top 10 best-seller lists simultaneously on The Wall Street Journal, USA Today and the New York Times.
His history in Wikipedia
Kiyosaki left the Marine Corps in 1974 and got a job selling copy machines for the Xerox Corporation. In 1977, Kiyosaki started a company that brought to market the first nylon and Velcro "surfer" wallets, which grew into a moderate success. In the early 1980s Kiyosaki started a business in which he licensed T-shirts for Heavy Metal rock bands but the business failed and he allegedly had to declare bankruptcy and ultimately became homeless.
Kiyosaki became active in a personal growth seminar, called "Money & You", which was started by Marshall Thurber. These 3-1/2 day seminars were taught around the US and Canada. The premise of the seminar was teaching the works of Buckminster Fuller and promoting the concepts of win/win and personal responsibility. When Thurber gave up the business in 1985 Kiyosaki took over the seminar business with Thurber's former partner, D.C. Cordova. They taught the course to a large number of students primarily in Australia and New Zealand. In 1994 at the age of 47 he shut down the business because of adverse publicity in Australia and "retired". Around 1996–1997 he formed Cashflow Technologies, Inc. which operates and owns the Rich Dad (and Cashflow) brand.
So he failed in business in eighties, started seminar business motivating other people and became successful. Good for him.
Criticism (Source: wikipedia)
Detailed analysis of public records (including SEC and county registrar of deeds) find no evidence to support Kiyosaki's status as a successful investor and businessman prior to the formation of his present venture, Cashflow Technologies, Inc. They claim that his wealth has come as a result of selling books and audio presentations about topics he has not personally succeeded in and that he is probably worth far less than the US$50 to US$100 million he once claimed in an interview.
Kiyosaki's books and teachings have been criticized by some for having anecdotal lessons, but lacking concrete advice on what exactly one should do. Many readers find his work highly motivational and educational, but some find it lacking information to put it to use. Kiyosaki responds that his material is meant to be more of a motivational tool to get readers thinking about money, rather than a step by step guide to wealth. He also says the books are supposed to be "interesting" to people, which precludes involving a lot of technical material.
Am I stopping you to read his books or causing it completely useless? HECK NO! They're what they are: Motivational material. They've been best sellers. But so are Lord of the Rings and Harry Potter.
They may motivate you and bring great success to you. But I'd rather read books with technical content on it, some thing more than "motivation".
Please let me know if any of above is not correct. I'll update it and I'll encourage you to update wikipedia as well if you have solid facts contrary to what is there.