I tried reading on the iPad in the park and it was really not that good of an experience. The glare was very distracting and sometimes reading became impossible, depending on the sun-to-shade balance. At home it's just plain awesome. I still use my eInk readers. Outside and for novels only.
Well, the iPad has its glare, so outside use depends on "where exactly" the outside is located
, but on the other side, try reading eInk in the evening or at night without lights on
...there's a gadget for that...