You may ask, what's the point of taking an extra turn if you can't untap? It would seem that the most you can do is activate planeswalker abilities an extra time. But, if you have Green mana, you could play cards like Awakening and Bear Umbra to get around that drawback.
Awakening is an Enchantment for 2GG which untaps all lands and creatures each turn. Bear Umbra is an Aura (Enchant Creature) that also costs 2GG and whenever the enchanted creature deals combat damage, you get to untap all lands you control. The most obvious solution in any case is to equip a creature with Sword of Feast and Famine, which does essentially the same thing as Bear Umbra but also makes your opponent discard a card – plus it's a colorless Equipment.
Planeswalker-based decks have become particularly popular in Commander and "Super Friends" decks have been around for quite awhile as casual favorites. As such, Savor the Moment has seen play here and there. But, considering how many extra turn spells exist in the Commander format, Savor the Moment is usually the forgotten one.
That being said, there are fewer planeswalkers in the format for Savor the Moment to take advantage of, but there are some powerful ones: Ajani, Caller of the Pride, Ashiok, Nightmare Weaver, Dack Fayden, Domri Rade, Jace Beleren, and Liliana of the Veil. Tibalt, the Fiend-Blooded is available, too. Getting additional effects from Ashiok, Jace, and Liliana seems worth playing Savor the Moment.
Unless you have an easy way to untap enough mana to be able to make that extra turn useful, Savor the Moment is one of those very niche cards mostly only useful to enable extra activations of planeswalker abilities. That being said, it's still a cool card. It's one to watch for, especially if you happen to be playing against a deck that's built around taking extra turns.