Here are this Charm's three modes:
• Target creature you control gets +1/+1 until end of turn. That creature fights target creature you don't control.
• Counter target spell unless its controller pays 3.
• Creatures with power 3 or less can't block this turn.
The first mode is very good, especially in the colors that this card would be played in. Red/Blue/Green decks tend to play a lot of aggressive creatures. So giving one of your creatures +1/+1 and having it fight a creature you don't control usually is going to be a win for your guy. It fits in perfectly with the Temur Clan's strategy and its Ferocious mechanic. That +1/+1 until end of turn could activate a Ferocious ability that wouldn't otherwise trigger, making one of your creatures with 3 power into a creature with 4 power until the end of the turn.
The second ability is essentially Mana Leak. There was speculation of that popular counter-spell seeing a reprint in Khans of Tarkir. Indeed, it was functionally reprinted on this card. It's an interesting effect to see on a RUG card, but useful nonetheless.
The third ability prevents creatures with power 3 or less from being able to block that turn. Pilots of super-aggressive decks will have to be aware that chump-blocking may not be possible if the opponent has access to this card. While that third mode may not be used all that often, it has to be anticipated now whenever a Temur deck is being played.
This is a very strong Charm that should have seen a good deal of Standard play. It did in fact see a ton of play right after the release of Khans up until the release of Fate Reforged, after which it pretty much disappeared from competitive play. Temur Aggro played anywhere from one to three copies of the Charm. Some Midrange and Control lists also used the card in varying counts as well.
While it saw very little competitive play after March 2015, it still sees play in a few Commander decks. Predominantly, it found a home in Surrak Dragonclaw decks, which makes sense considering that he was the Temur leader from Khans. The synergy seemed pretty obvious in building a deck utilizing all of the best Temur cards from the block. Other Commander decks in those colors like Yasova Dragonclaw (from Fate Reforged), Animar, Soul of Elements, Riku of Two Reflections, and Intet, the Dreamer have dabbled with Temur Charm.
A card that gives the option to serve as removal while also potentially activating Ferocious, act as a permission spell, or open up the floodgates for a lethal attack is quite versatile. While its days as a useful competitive toolbox card are long past, Temur Charm still has a home in the right Commander decks.