Between the 505 and the cybook, the 505 is definitely the better choice. The grey scales on the cybook makes most pages seemed overbrightened compared to the larger range of the 505.

As for ease, I find the the CBZ\CBR function in Calibre works very well and is basically a point/click/use solution.

The main problem you will have will deal with the source material itself. Not only are you reducing the size of the page you are reading but also reducing the sizes of the fonts used on the page. In many cases it will become harder to read than before with some fonts becoming completely unreadable (fonts that started out very thin to begin with will be nothing more than stray dots after being reduced).

Right now for manga reading using a dedicated E-ink reader would be:
Iliad>505>Cybook/Kindle (Iliad wins due to large size and good greyscale but is very expensive compared to other options.)

Another option to look at is the new set of mobiles called "netbooks". These are smaller and cheaper than standard notebooks, have very good resolution and screen sizes and can run pretty much any reader or viewer program out there. They are NOT as pocket portable or as long lasting between charges as a dedicated reader but are none the less a viable alternative. (I'm currently using a friends Acer Aspire while he's in a training lockdown and find that it would be an excellent complement to my either my Sony or my Cybook. The dedicated readers are better for text reading, the netbook for graphics and general use. (Though I will admit that lying on the couch with the netbook open on your stomach and a wireless mouse to scroll though the pages IS pretty nifty. Lazy but nifty.)