Leif is generally the best buy, as demonstrated by the mixes in leaderboards in tournament. This may change over time, though. Essentially, Narlax may be a good addition if you have fully-awakened Fee (teleport arrows, though some mixed reports of how useful this is) or fully-awakened Smoudler (permanent haste with Narlax), and can pair this appropriately with a blessed Fee, Smoulder, or Narlax. This is about 20% of blessable heroes, so in theory she can really help out in tournaments.

Tournaments aside, she is one of the most fun heroes to play. However, like others have mentioned, she is fragile as compared to Leif. There are some levels that are difficult without meteors without Leif (avoiding spoilers).

Currently, Leif is one of the dominant heroes used on Tournament. If you want to cruise through the levels, Leif in my opinion is still the best (though if you're having a hard time with split section levels, Narlax is great because you can teleport around).

In tournaments, at the point where being able to suck in all monsters to a single point is really useful (for hitting with a meteor), Narlax dies too quickly to be of consistent use.

All this said, there's supposed to be a new hero in 1-2 months. The devs have this somewhat obnoxious trend of making the new hero much more powerful than all previous heroes (though I think they did a better job with Caldera as compared to Leif, chest bug aside). So if you can hold on for another couple of months, there's chances that you'll be happier having held on and buying whatever monster hero comes out next.