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This is a complicated question, because there are many cases and each would require a calm evaluation to grasp the causes, methods and impacts. ETA, IRA and Al Qaeda are not the same and grouping them in a single answer would just create confusion.

Since going case by case is in my opinion the best approach, I mention one particular case of terrorist that achieved exactly what he was hoping to accomplish. Yigal Amir assassinated Yitzhak Rabin in November 4th 1995 and effectively destroyed the Israeli-Palestine peace talks, which was his goal. The right wing parties in Israel seized the moment to organize a coherent political response and were able to beat the left wing government on the pools, they have been in power ever since and no peace process came close to what Rabin achieved.