I spent about a week implementing and evaluating both DownloadsII and
Links and Downloads Manager. I really like the layout of the DownloadsII file area better than LDM. The presentation is better and it's more intuitive. But the absense of a progress bar made it a choice I couldn't go with. LDM has a very well implemented progress bar for uploads so the user's browser doesn't just sit there churning with no feedback during large uploads. In addition, LDM's developer is more responsive in his forum. Of course, DownloadsII's developer does this for us all for free so it isn't fair to complain. But (at least at present) it seems LDM is better supported.
If DownloadsII ever adds a progress bar like LDM's I'll probably switch over.