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Word of Seizing says:
Untap target permanent and gain control of it until end of turn. It gains haste until end of turn.
Can I take control of any permanent? Specifically a planeswalker?
I remember somewhere the rules said that if a portion of an effect is not legal or possible, it gets skipped and the rest of the ability resolves (barring any specific portions of effects that say explicitly otherwise)
Yes, you can target any permanent including a planeswalker. You're right that impossible parts of an effect are ignored - in the case of this spell an example is that you can still target a permanent that's already untapped. Untapping something that's already untapped is impossible, so you just don't and move on with the rest of the effect.
However, the part about gaining haste isn't actually an example of an illegal or impossible effect: the planeswalker, or whatever else you steal, will get haste. Most of the time this will be irrelevant (haste doesn't do anything on a non-creature), but if the permanent later becomes a creature it will matter. So if you steal my Gideon, Ally of Zendikar and then use its +1 to turn it into a creature, you'll be glad this spell gave it haste, because otherwise you couldn't attack with it.
