As someone accustomed to selecting the location with every download, I really don't like the idea of a separate folder just for downloads. The first time I used a browser like that, when I clicked a link and it downloaded the file immediately, it was quite disorienting because I didn't know where it went. The files I download need to go in different places depending on what they are, so it's hugely irritating to have to go to Downloads (which is conveniently somewhere with a nontrivially long path...) and move them to where they should go. This encourages users to put everything they download in one place, which is really bad for organisation.

IMHO the browser should always ask where you want to save something, defaulting to the last location you chose. The extra click/keypress, and having the files downloaded to their intended location, is far better than the multiple actions required to rummage in Downloads and move the files to where they should be afterwards. The workaround, to right-click and "Save As...", also involves an extra action.