I have the Sony PRS-505 which I also agree is the best of the bunch. However, this year my Kindle DX has become my reader of choice because I love the larger screen -- more words per page turn and larger choice of fonts.
If I were to consider another reader, it would have to have a larger screen. I recently went back to my Sony because I felt guilty for neglecting it ... but it seem so .... small (I can't even imagine reading on a 5" reader!) despite still loving the screen (best one on the market!) and the folders and interface and the beautiful red brushed metal.
I have to admit that the siren call of the new Irex reader is ringing in my head and when that is released, I will be sorely tempted. I'm sometimes tempted to buy a refurbished DX as an emergency backup reader -- the thought of my reader not working sends cold chills down my spine! But being unemployed and with money particularly tight, my smart money angel is kicking my bad money devil's proverbial butt!