I bought a Sony 505 way back in December 2007. When Sony stopped making them I bought a back-up unit. The original reader is still chugging along despite years of abuse. The damn thing just won't die. The back-up unit is remains in a cupboard, unused.
I got an iPad 1 after seeing them in a shop display when they first came out. It was love at first sight. I keep it in a bulky but very rugged Capdase leatherette case. The case cost about $100 in Thailand but I've never regretted buying it. My iPad travels everywhere with me. The case has acquired a few scars and stains but the iPad remains pristine.
Given a choice between reading an ebook on the Sony and the iPad, I choose the iPad. My eyes are getting older and the adjustable brightness and size of the screen is a huge help. Also, I spend a lot of time on planes and in darkish restaurants and hotel rooms. The bright screen is a big help there too.
It's a bit fiddly loading ebooks onto the iPad, but no worse than the Sony and of course there's the Amazon and Borders apps (and others, depending on your country). Remember, when you buy an iPad, you're buying a Kindle as well! And of course, if Amazon doesn't have enough books for you, you can sideload mobi format ebooks into the kindle app.
Also, unlike a dedicated Reader, the iPad is capable of doing a whole bunch of other stuff.
As you can tell, I'm very happy with my iPad. I am now looking longingly at the iPad 2.