I can't answer specifically for the two sites mentioned above however generally, it's working brillliantly here
(well for the funcionality it currently has, which is enough for me). If you download the ebook creator software from the main mobipocket website you can convert any .txt or word format ebook to a .prc which I have done with 2 of my ebooks and it has worked brilliantly (unfortunately it's only a trial software so will have to pay for it unless I can find a free converter). If you do a google their are converters out their to convert other ebook formats such as .pdb to text so you can then convert any current ebooks you have to a readable format!