For three mana, you get a creature with plenty of upside.
First off, it’s a 2/4 body so it is more resilient than other “support” cards that can be generally weak and easy to remove. At the very least, you can use this centaur lady to ward off a lot of dinky attacks from your opponent.
The Courser of Kruphix‘s first effect of being able to play with the top card of your deck revealed is both a blessing and a curse. Though you are able to see the next card you are going to draw next so you can plan ahead, your opponent gets to see the upcoming card as well and thus can prepare for it.
This means the Courser of Kruphix takes out some element of surprise from any deck you decide to run her in. This is the only real “flaw” of this card in all honesty, but this advantage/disadvantage is well worth it for the next two effects this card offers.
The ability to play a land card from the top of your deck is huge. Though you are not necessarily getting ahead in the mana curve (because it still counts as your one land drop per turn), this effect is indirectly pseudo-card draw.
Assuming you are playing a land card from the top of your deck at least every other turn, you will be able to a) keep up with playing your land drops and b) you are sifting through your deck for the cards you need to help you win the game.
Being flooded with mana can spell defeat in some games of Magic, but Courser of Kruphix cuts down on those off-turns where you don’t necessarily want to see land. Instead, ideally anyway, this centaur is making sure you have plenty of land to cast any spell in your deck (preferably ones with high mana costs that grant powerful effects.)
Finally, the effect of gaining an extra life point for each land you play may not seem like much, but it will definitely add up over time (especially if you happen to have more than one Courser of Kruphix on the battlefield because the life-gaining aspect would stack.)
All in all, Courser of Kruphix is an excellent card for three mana. Just about any deck in Standard that runs green should definitely run a full set of her. Her card value has already risen up to the $7+ mark. If you happen to come across this centaur, be sure to recruit her to your ranks.