I've been using River for over a year now. It's improved over earlier versions but it'st still a "proof of concept".

It still doesn't use the default browser and only uses Safari.

It still doesn't open the sites in tabs for browsers that support tabbed sites (most do now).

It still doesn't allow you to set what sites you want checked in a preferences file.

It's really a jacket for an Applescript to go to a list of sites in Safari and open them with a search option. Some of the sites on the list are closed and require membership in order to search their archives.

Again, still not worth it.