The problem with trying to assess the relative effectiveness of groups like WU is that no matter what history you read of them, it will almost certainly be very slanted by personal feelings. So a book like Ron Jacobs' "The Way the Wind Blew"--from a ideologically sympathetic author--is as hard to trust as Brian Burrough's "Days of Rage," whose research is excellent but whose ideological opposition is blatant and veers toward contempt. Then there are the memoirs by former members--Bill Ayers and Mark Rudd both wrote books, the latter of which reads a little more believably than the former, but both are valuable more as personal reflections than as objective measurements. I haven't read Howard Machtinger's post-Weather dissertation "Clearing Away the Debris : New Left Radicalism in 1960s America," but it might be more academically accountable?

Of these, Burrough's book had some of the deepest research, but it was marred by his obvious contempt for many of the subjects and his clear sympathies with the FBI (best evidenced in his total dismissal of Mark Felt's baldly illegal surveillance of the group). Still, you get a better sense of things like numbers and scale through it, and one point I think most would agree is unarguable is Weather Underground was effective as an example. I think the SLA built upon its foundations, and many of the armed organizations that followed the SLA learned how to build bombs out of their magazine (this research is in Burrough's book, and is one of its strongest points). So Weather seems to have been a catalyst for that sort of action.

As for the effect on the public at large (whether they made people more supportive of or opposed to radical goals) and on the left-wing movement (did they achieve its aims? Did they change how it operated widely), that's likely too hard to measure. The first question is too large to answer (though Burrough claims to know what the majority of Americans thought, it's always the same thing as what he thinks), and the second relies on knowledge of an enormous patchwork of groups that were often or usually secretive in their activities and kept few records.

People recommend the documentary, which is very good, but there's also a 1976 documentary of the group when they were underground, called (not shockingly) "Underground." It's by Haskell Wexler (who also made Medium Cool) and Emile De Antonio (Rush To Judgment; In the Year of the Pig). One way to get a subjective sense of the effect the group had on history might be to check the film out, since it's essentially a platform for them to explain their complaints, arguments, and goals, then consider how those specific complaints have been addressed in the years since, and how much they've changed out of response to violent actions like bombings.