Of course the challenge is to define what "success" is, and also to determine from who's point of view you are determining success.

I study left-wing European terrorism of the 70s. For groups like Germany's Red Army Faction, it would be hard to argue in any way that using terrorism accomplished anything at all resembling their goals. In fact as the tactic proved to be a failure, they kept subtly changing their goals.

Overall, their goal was to start a socialist revolution, and inspire the "oppressed masses" to join their cause. But after their initial wave of bombings and murder horrified those masses, and after the leadership of the group was captured, the main goal was simply to get them released from prison. So terrorist actions, such as the 1975 takeover of the West German embassy in Stockholm, were done to secure the release of the imprisoned leaders. And after THOSE tactics failed to work, the remaining group members used terrorism to attack symbols of the state; the goal simply to be a thorn in the side of the military/capitalist system.

You can, in fact, easily argue that the exact opposite of their goals was the net result of their efforts. The RAF ultimately created a society with a much more conservative power base, with a many new anti-civil liberties laws in place, a much stronger national police force, etc...

That said, one could also argue that the provisional IRA's use of terrorism was a successful tactic in the sense that it helped keep alive the struggle for many decades. They felt, and were probably right in thinking this way, that there was no real way to fight the brits as a traditional army; but they could certainly wear down their resolve through terrorist tactics. I could be argued that this multi-decade strategy, and a clear willingness of the part of the provisional IRA and other groups to continue them in perpetuity, was a driving factor in the eventual peace.