Certainly pixel density is a different strokes for different folks thing. It's why I sent my Nook Tablet (160ish ppi) back to wait for this more than 6 months ago. It's not unreadable even to me, but it isn't comfortable either. When I moved from my 140ppi TX to my 260ppi TP2 (both were my primary reading devices), I was spoiled beyond repair.
I don't know for sure what my minimum pixel density for a tablet would be, but right now I'm shooting for 200+. The N7 has 215ppi.