[annalex]

The LXX is useful the same way the Targums are useful--the translation choices give us an idea of what the rabbis before the time of Yeshua believed about numerous verses. But it's hardly a sacred translation

How about I let you convince the Orthodox of that, and I'll watch?

[Buggman]

I assume you mean the Eastern Orthodox rather than the Orthodox Jews (the latter wrote off the LXX in the second century or so). And I'm sure that it'd be every bit as fruitful as arguing the Immaculate Conception with a Catholic.

But, just because I believe in making a point, here's 2 Co. 8:15, quoting Exo. 16:18 (pardon the English transliteration, but I don't feel like trying to render this in unicode),

O to polu, ouk epleonase, kai o to oligon, ouk elattonese.

And here it is in the LXX. I'm quoting the whole verse and bolding the relevant portion.

Kai metresantes tou gomor ouk epleonasen o to polu kai o elatton ouk elattonesen ekastos eis tous kathekontas par eautou sunelexan

Hmm, some of the words are the same, but not all, and the construction is different--in fact, Sha'ul's is actually closer to the Hebrew in word order. It's clear that there were times when Sha'ul (Paul, if you prefer) deliberately did not use the LXX--in fact, while he quotes from the LXX 51 times, he renders his own translation 38 times and creates a translation closer to the original Hebrew than the LXX at least four more times (one of which is the quote I used above).

The Septuagint was a useful and widely-used translation in the first century, much as the KJV is today, so it makes sense that Sha'ul and the other Apostles would make regular use of it when quoting the Tanakh for their Greek audience, but since they also made their own translations from the Hebrew text, the idea that the LXX was a sacred translation to them, as good or better than the original Hebrew, doesn't hold water.