Last block I collected was Mirrodin as well, though I've played with other folk's decks since.
The Planeswalker abilities like Gideon Jura's are pretty simple; there's a big number on the bottom that represents the number of Loyalty Counters with which the card enters play. Once per turn you can activate one of the Planeswalker abilities as a Sorcery; the ability takes effect and then you add or subtract the indicated number of Loyalty Counters. If the card runs out of counters it goes to the graveyard.
on edit: Damage to Planeswalkers hits their Loyalty; two points of damage from a blocker would remove two counters from an attacking Planeswalker. An opponent can choose to attack a planeswalker rather than you; your creatures can block the attack.
Planeswalkers aren't actually creatures; they're powerful allies who will work with the player until they get sick of it, basically. Think of them as another player working alongside you. Managing their loyalty adds a nice dimension to play.
There's a Legendary-type rule attached to the cards; if multiple Planeswalkers of the same name come into play then they all hit the graveyard.
I don't recall Boomerang, actually.
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