I own a Nexus 7 and an iPhone 4S. I don't have an iPad to compare to, so I don't know about that. My subjective opinion:
I find that the text looks pixelated and it's taking some time to get used to it. I'm not sure how much of it has to do with Android's font rendering or how much has to do with PPI. One thing is that since I can hold the Nexus 7 easily with one hand, I often ended up holding it as close to my face as I do my iPhone. It's a bit unfair as the iPhone has a PPI of 326 and Nexus 7 is 216. So of course text on the iPhone will look better.
My other issue is that the screen is too bright, even at it's lowest brightness setting. Using the Nexus 7 in low light or a dark room is uncomfortable due to this. I don't know if this can be fixed in software or not.
That said, for a $250 device, I think the hardware is great. My issues are more with the user experience of the software, but that's another thread.