Can we trust the Septuagint? I've previously used the Apostolic Bible for digging deeper into the meanings of passages.  However, I've found that some passages in the Old Testament make no sense compared to the modern translations.

I figured out that the problem was that the Apostolic Bible was using the Septuagint ("LXX") version for the Old Testament.  (The Septuagint, for those who don't know, is an ancient translation from the original Hebrew into Greek.)

Once I realized this, I stopped using the Apostolic Bible out of fear that it would lead me down the wrong path.  Now I'm curious how valid is this fear?

So, the question is:  Can we trust the Septuagint as a valid translation of the original Hebrew?  

Alternatively, what can we learn by using the Septuagint compared to going straight to the current Hebrew editions of the Old Testament (such as the Online Hebrew Interlinear Bible)?
The Masoretic (Hebrew) text is closer to the Bible Jesus used, but Septuagint (LXX) was the one used by the New Testament writers.

In most of the Old Testament, the differences are not very significant. However, in a few places the Masoretic and LXX have sharp differences. For example:


  Isaiah 7:14 NRSV from Masoretic Text: Therefore the Lord himself will give you a sign. Look, the young woman is with child and shall bear a son, and shall name him Immanuel.


Many modern Bible translations prefer the LXX in passages that are quoted in the New Testament:


  Isaiah 7:14 ESV from Septuagint: Therefore the Lord himself will give you a sign. Behold, the virgin shall conceive and bear a son, and shall call his name Immanuel.


When this verse is quoted in the gospel of Matthew, it is clearly the Septuagint version:


  Matthew 1:23 NRSV "Look, the virgin shall conceive and bear a son, and they shall name him Emmanuel," which means, "God is with us."


For that reason alone, I'd be reluctant to dismiss the Septuagint entirely. The ideal (short of learning Hebrew and Greek) would be a translation with lots of footnotes to indicate where the Hebrew and the LXX disagree, and explanations of why the translators prefer one or the other. Failing that, it's beneficial to have several translations available to see how different translators handled these passages.