I use both a 15" rMBP and iPP. Carry both with me every day.
This is my task split:
- iPP:
- Browsing
- Email (Google Inbox)
- Taking notes in class (currently doing a PhD)
- Written homework (I've completely given up paper at this point)
- Studying
- Code reviews (I can kick back on the couch and look at colleagues code on Github and comment)
- Meeting notes
- Playing games
- Arguing on the Internet (doing it right now!)
- rMBP:
- Programming (my PhD is in Computational Nuclear Engineering... The balance of my research involves development of massively parallel codes for using supercomputers to simulate nuclear reactors - I won't be doing that work on an iPP any time soon)
- Journal article writing (Latex is still better done on a real laptop at this time, but it's getting better on iOS)
- Presentations (PowerPoint, Keynote and Beamer... I'm not ready to do those on my iPP)
On balance, I use my iPP for many more hours during the week... but the rMBP still does essential work. I don't regret carrying both, because they're both awesome at their particular tasks... So I have no compromises.