Aside from assigning each creature an alignment and converting AU caster levels to D&D ones (ie, a creature that casts spells as a magister casts them as a wizard), there's not much else you'll have to convert. All the AU-specific feats and spells are given in an appendix.
I have Legacy and love it, absolutely love it. I still havnt finished reading all the monsters yet, and have already in my campaign used two creatures, and have plans to use at least 3 more.
All the entries have good flavorful descriptions and portraits, and most have intresting mechanics. The CR ranges are a good solid mix, generally starting around CR 3 up to 20, with most hovering in the middle.