The only reason I upgraded from my PRS-650 to the T1 was precisely because I could root it
The ability to run apps is fantastic. I can run the excellent BBC News app, RealCalc, ConvertPad, Kindle, aCalendar, Wikipedia Pro, Tube Master (which has saved me from a lengthy travel delay more than once now), PowerAmp etc. I can even check my email.
I used my 650 daily to read books, but had to carry around my tablet to do other stuff. I can now just carry around my T1 most days.
I've gone from just an eBook reader to something much more useful.