I bought P7 as soon as it was released so it's been a while.   I got P8 because I liked the added feature of IDL and the fact that they fixed the renderer to fully utilize all cores really speeds up renders for me. 

The upgrade price is the same as P7's was a couple of years ago, so that was a positive for me as well.   It comes out to less than 100 euros - a week's shopping at the supermarket!  Plus I hardly bought any content for about a year and rewarded myself for it :)