Initially I too was skeptical about planeswalkers. Seeing the first one and finally finding out the rules governing them, I was left with a bad taste in my mouth. Four to five mana is way too much to be investing in a card that cannot provide an immediate effect and is potentially more vulnerable than a creature, damage wise.
This was before I stopped thinking of them for their giant effects and started closely examining them for strict card advantage. They trade with more than one card on average every time. If they don't, its because your opponent attacked them, which at least provides an extremely expensive fog for yourself. If you can protect them, these guys will net you solid 3+ for 1's every time, and as we know in Magic, everything is determined by tempo and card advantage.
Naturally, I find that planeswalkers like Garruk and Chandra have found ways to affect the board position potentially and net instant card advantage for you, while contributing to protecting themselves.
That last part is key; if the 'walker can protect him or herself, they are that much more powerful. It's why I cannot see a solid deck with Liliana in it. She drops on 5, tapped out, and at the very least sets up the poorest 2-for-1 yet (the +1 discard effect) or sets up the top your deck. I might be underestimating the tutor power, but if the opponent has a creature + burn (I'll assume you used Damnation the previous turn, she likely goes to 1 if you added to her and 0 if you tutor, The tutored card and a burn spell can't be worth 5 mana (mini fog as well, yes). Still, I might be vastly underrating her.
Bottom line, I am starting to get a hang for the planeswalkers and where they might be played. Certainly they don't go into any and every deck; they cost far too much in mana and subsequently tempo to be worth playing in anything but mid-range and slower decks. But if you plan on controlling into said mid-game or longer, why not eat up your opponent's time and resources with the threat of a card advantage machine like Garruk?
