ok, I have been experimenting with my ipads and the iBooks and PDFs and i have some recommendations to make.
1) 100% of the time drop box works flawlessly on letting you load your ebooks into whichever app you wish to use. Marvin is the only one which lets you load multiple books at once from drop box into Marvin. Marvin only works with ePub and no other format.
2) sync can work on the ipad but it is quirky. one thing it does consistently is turn off my selection of sync all ebooks back to sync selected ebooks, this is a royal pain. what i do is move the books i want to sync from caliber into iBooks on my mac air. then i make sure iTunes is set for sync all books under the books tab for my ipad. it will sync wirelessly if you check the box allowing the two to sink wirelessly.( look at your summary page for your ipad in iTunes for box).
3) you don't have to go find the file to move a book from calibre. you can just drag the book cover icon over into iBooks without holding any extra keys. you can move it into dropbox too but you have to hold the option key so a duplicate moves to dropbox. when you delete these books from iBooks and dropbox, it does not delete them in calibre.
4) overall i still prefer using goodreader for PDFs. you can either drag and drop them into the goodreader app in iTunes or you can load them from dropbox.
5) i have not been able to load into either iBooks or Marvin from calibre itself despite having installed plugins for that purpose for both. i had no trouble loading my husband' s ebook reader, non apple, from calibre. his worked perfectly. his device loaded onto the toolbar immediately and i just dragged and dropped files onto his device icon and they all loaded. I cannot get my ipad to show up on the caliber toolbar in any way. I have read tutorials about it but they never work.