I had a first-gen Kobo. Advantages: e-ink technology is easy to read and the screen is matte-finished so easier to read outdoors. Also nice and light. Disadvantages: slow as molassess to load and turn pages. Buttons felt very cheap quality.
Currently I use an iPad2 and love it. Yes it's pricey but it can do more than just hold books so when you get tired of reading it's easy to flip over to a game or browser. Shiny screen makes reading in sunlight difficult, but I don't tend to read regular books in the sun either anyways. Plus with the iPad I am not limited to just buying books from one location.
Had I not gone with the iPad I would have bought a Kindle product as friends of mine have them and are very satisfied with them.