Both functions add value to the already free to download clients. The download manager provides a nice touch for organising the occasional file. It's a nice option to have but I've never been one for doing web downloads on my phone, but then I have a desk bound computer for heavy lifting like that!
Opera Link is actually far more useful. One of the 'delights' of reviewing Opera Mobile on multiple smartphones is always typing in the same URL's, especially when you have Opera's quick access pad of nine thumbnailed bookmarks. No more! Just set it up on one phone, run Opera Link to sync it to a central database, and bingo, you can move to another handset and flip the switch in settings to have everything copied over.
A nice touch, that works well.