Jenara is a decent card. It doesn't strike me as something I wouldn't leap at the chance to put it in a deck, but I wouldn't turn down an opportunity to easily acquire one either.
That said, Jenara isn't a bad creature for three mana. 3/3 with flying is usually enough to hold off your opponents' attacks until you can cast your more powerful spells. Jenara's ability to place +1/+1 counters upon itself is what makes it a mythic rare, among other things.
To conclude, Jenara, Asura of War isn't a card you'd want to build a deck around, but it's helpful as a defender for early on.
