I like the idea behind those sites, automating the lending features of various ereaders, the Kindle notably.
I can't imagine that the Kindle team didn't imagine services like these springing up the day after they implemented the lending feature. But I do wonder if they imagined the size that these services are capable of growing into. It's now almost trivial for the operator or anyone really to alter the flow of these lent books into a DRM stripper for reupload.
But in theory, this should not even have to be done. Type in what book you want and have it lent into your ereader. Pay what you will when you can.
I fear the creators/publishers will soon turn off the lending feature of their kindle books, but maybe my fears are unfounded, only time will tell.