Well.. When it comes to EGW here's my take on it. In her writtings there are many accounts of visions given to her by God( or what she and other's living in her time says were from God - for the skeptic). She uses the phrase "I was shown" thousands of times in her writtings.
Now either
1. She "was shown" from God and therefore we had better think carefully before speaking against or rejecting her council.
2. She was seriously Deluded - mental illness of some description.
3. She "was shown" from a power not of God.
Considering there are surrounding EGW many accounts of supernatural occurances eg. visions, knowledge about people that ordinary people just shouldn't know and prophetic utterance. For me I would personally rule out option 2. which leaves inspired by God or inspired by Satan.
Having read a reasonable amount of her writtings I am constantly amazed at what she wrote and I am often inspired to seek God more earnestly and get a real blessing from what she has written. I've never had the impression that she tries to usurp the authroity of the bible but only tries to open our blind eyes to see the glorious light of the bible.
This is how I view the idea of being an adventist and rejecting EGW. Adventism has from its birth a woman who claims to have been shown many things from God, most of the founding members of the church also believed EGW was a prophet who recieved light and knowledge from God. So if these people were all living a delusion, why accept the things and be part of a church that they built? (under Christ of course). The foundation of Adventism would have some serious structural defects.
As for EGW replacing the bible? just read what she wrote herself, there is a great chapter in GC on this point alone. Called "The scriptures a safeguard"
http://www.whiteestate.org/books/gc/gc37.html