I would say, that yes, she is still very important to Seventh-day Adventists, a central figure in our faith and beliefs. However, one does not have to believe in Ellen White, per se to become a member of our church. What is important is that one believes in prophets as a whole.
The best suggestion I can give you, if you are truly interested in learning about Ellen White and her life, would be to visit whiteestate.org, that is the official site. You can access all of her writings on that site as well.
There is an excellent DVD on her life, titled "Meet Ellen White", which is available for ordering on Amazingfacts.org. When I first started studying the bible and had decided that I wanted to join this church, I received this DVD as a gift from a friend, and I was and still remain extremely grateful for this gift. This DVD presents the biography of Ellen White, her story with a "no slant", and that is what everyone should have, her life story with no pressure as to what you will get from it or what you will believe when you are through watching it.
Just as with anything else in life, you will find some who put way more importance to Ellen White than they should, and I say that knowing how Ellen White herself felt about that. She wouldn't like a lot of "fuss" being made over her. She was a messenger of God, it was her duty to pass those messages along, and she did that. Mrs. White saw herself as a "lesser light leading to the bigger light". She couldn't have stressed the importance of reading and studying the bible strongly enough. The books I have read so far, Steps to Christ and I'm working on The Great Controversy right now, have gone hand and hand with the Bible. I have found nothing in her works that contradict God's word...SO FAR. I say that because I can't speak for all of them, there are MANY writings to get through, and I couldn't honestly speak for the rest of her works. I can say, however, that I don't expect to find anything different than I have already.
The first thing that struck me was how well she wrote, how she brought things to life, and with only a 3rd grade education. I have heard it bantered about that she plagerized her works, copied off of other writers..but I have yet to have anyone who has allegated this just WHAT other writers she copied from, just WHO were they?
In conclusion, from the limited experience I have had, how much credence, how much importance a Seventh-day Adventist puts on Mrs. White's messages and writings, is an individual thing.
Our main belief and focus remains on Jesus Christ and His soon return. He receives all of the praise and the glory in our church, it is God's teachings that remain our solid foundation.
Thank you for taking the time to visit and ask questions.
May God bless.