I guess it's a matter of whether the OP minds doing the conversion before reading the book. I have a Kindle so I get .mobi and .azw books that I can read right away. Every so often I convert a bunch of them at once for archive. When I want to be reading I can be doing so right away. It doesn't matter where I am and if I have conversion software with me. It's a small distinction but it may be relevant to the OP, I suppose.
Of course, since I usually download my books over the EVDO connection as I want them, I'm more likely to be getting my books while away from my house. If I had a Sony or a Cybook, I would probably be downloading them to my home computer where the conversion would only add a minute or two to the process.