First off, to compare this recording (made when Hahn was 16) to Milstein's Sonatas and Partitas (made in the prime of his career, and considered one of the 3 or 4 greatest violin recordings of all time), shows how much I think of this one.
Now to be fair, Hahn's performance of the Second Partita is not Milstein's. Milstein's Giga is much fiendishly faster (played without the repeats, as was common in the early 1960s -- Glenn Gould in his piano recordings of Bach always left them out too), and a true baroque gigue. And Milstein's Chaconne! -- well, it is always on the edge -- technically, and in the depths of human emotion, suffering, and transcendance it engenders in the listener. And this is despite the fact that he plays it faster. It is almost impossible to come away from the Milstein recording of the Chaconne without feeling both emotionally exhausted and exhilarated. This remains the definitive performance.
Having said that, the Hahn performance will always have a place in my heart. The sound is rich and full, not ascerbic and vibrato-free like Milstein, the phrasing is delightful (no one could ever claim she hasn't thought through, note by note, what she wants to do); the rubato is judicious. The mellower Chaconne makes it in some ways more enjoyable (you'll never want to sit through listening to the Milstein twice on the same day! but you can repeat the Hahn again and again), though the highlights in this recording are the Sonatas, not the Partita.
I have bought and given copies of this recording to budding young musicians as inspiration. (I'd only dare give the Milstein to those who are emotionally prepared - it should be "R" rated.) Hahn will be around a long time -- she'll get to do it again (Milstein's famous one he did when he was 54, I think, and recorded them again (less well) in his late 70s.)
Buy it. Put it away. Be prepared to take it out again to compare it to what she does when she's 40.