I like backpack.tf because I can click on an item in a person's backpack and immediately look up item history, outpost, Tf2 pricecheck, etc. See the person's bp history, if they are marked SR scammer or VAC banned, hotlinks directly to their Outpost and TF2TP trades etc. etc. Also look at little graphs and charts of their bp and easily filter items for whatever that's worth.
There is way more functionality to backpack.tf than just pricing :\
As for slow load times other people complain about this but I have never noticed it the site is always fast for me...