I'm buying it retail strictly because I want the DVD, I have HL2 and EP1 on DVD, and i'm not going to miss EP2 on DVD. It'll probably be $90-100 but I really do not care, I pay the same amount for any other game every couple of months, and i'm getting 3 in one package.
While it is a great deal cheaper on Steam I'm just not willing to pass up an actual, physical copy of it in case of.. y'know, problems. Waiting 8 extra days is no problem either.