My personal opinion of them has always been that whilst they proclaim to be a sort of online morality police, they’re mostly in it for the giggles, or maybe I’m still annoyed about them taking down the PlayStation network…
either way, whilst I’m probably morally aligned with them, I can’t condone them, they’ll attack organizations for perceived attacks on free speech, when it suits them, they’ll also attack people for saying things (via their websites) that they don’t agree with – an unfortunate side effect of free speech is that it applies to almost everyone as long as they stay within the law, not just people who agree with you.
As for the disruption, they don’t often last long, but that’s not always their goal – whilst the DDoS gets the headlines, they often use it as a cover to steal data – last time they attacked the KKK they published a full member list.