When we talk about Christianity we talk about somebody who us following the example of Christ. Christ honor the saturday and congregate himself on synagogue as it was His costume, Luke 4:16 ,and as the apostle Paul did too for those who said that the sabbath was abolish after the crucifixion (Act 17:2) , adventist follow Christ in living by faith though the Grace of God the law that was going to be wrote in the hearts by God. Steps to Christ presents this concept very clearly. Adventist keep their bodies clean and healthy as Temple of God as 1 corinthians 8:19-20 ask us to do by obeying God's command in regard to clean meats and vegetarian diet given in Genesis originally and follow by Daniel and his friends. Adventist are christians because as Jesus said I have not come to destroy the law but to fulfilled and as Jesus said to us Example I has given so you can follow; Jesus did not need to get baptize yet He baptize to give us example; the same goes to the fulfilling of the Law. We are to fulfilled by Faith in Him though the grace of God knowing that he puts in us the desire as well the will to do it, however we have to believe and act upon by faith. In Short Seventh day adventist are Christians because Jesus told us that in the Scripture you might think that we have eternal life and they are which give testimony of Him, and if having eternal life is to know Jesus and the father then knowing the Scriptures and living by every word that comes out the word of God is as paul said looking through a glass the glory of God an being transform from Glory to Glory by the spirit of God. In short SDA's are Christians because our hope is found in Christ and his word and not in the perceptions that many has given the scripture founded in self-righteousness or for another hand cheap grace which are at the most basic level an expression of selfishness.
Seventh-day Adventists do differ in doctrines and beliefs compared with other churches, denominations and sects. At least the read and study the whole Bible for its beliefs and doctrines. SDA would be more related to the New Testament Christian Jews. Yes, there is much emphasis put on Ellen G. White for its doctrines, but if studied carefully much can be supported by the Bible alone. I am SDA, and I have not read her books since reading her statement that "if we studied the Scriptures the way we should have, we would not need her testimonies." (my paraphrase).
The thing about Christianity is that it has so many sects whose translations are considered to be correct by those who follow it. The same goes for the Seventh-Day Adventists. There is no difference between how right they are and how right Roman Catholics are. Just different sects is all.
Seventh-Day Adventists follow many of the same practices of other Christian denominations they simply believe that the day of rest was attributed improperly. People shouldn't assume there is something inherently wrong with the denomination or assume they aren't Christian solely based on the fact that they have church on Saturday instead of Sunday.
I do not think Seventh-Day Adventists beliefs accurately reflect Christianity. Seventh Day Adventists emerged from a movement called the Millerite movement in the 1840's. They believed that the second coming of Jesus Christ would occur on October 22th, 1844. When this did not happen they suffered a great disappointment and many followers disbanded and went back to the former major denominations.
I do not think that Seventh-Day Adventis beliefs accurately reflect Christianity. I think that while some of their beliefs are similar, I think overall, a lot of their main ideologies are quite different in nature. I also think that a lot of Christians might have issues with Seventh-Day Adventists and their ways.
The ideology of the Seventh-day Adventist is far an away a district break in Christianity. Death being "soul sleep" and that the evil will not go to a hell is wholly different than the rest of the Christian ideals. Overall the "Spirit of Prophecy" is another break away form the Christian faith and poses an interesting idea of continuing and authoritative source of truth. It may have its basis around Christianity but it is a clear break from the nominal faith.