Enjoy, I had one and in terms of IQ its better than the F/2.8 IMHO. I use the F/2.8 at the back of church weddings and it does help but for outdoor portraits then its value for money!
That was my reasoning. I have no need for f/2.8, so the extra bulk and cost of a second hand 2.8 IS mk1 vs the f/4 IS was pointless, as the f/4 is thought to be sharper than the mk1 2.8.
Justifying it to myself! Most expensive lens I've bought yet...