Heart is obviously at its worst if you're trying to win a quick scuffle over a flag, and it'll get locked if you lose. However, there are lots of other situations where heart is definitely a life-saver. One of the most important is to quickly recharge after a tough fight. I'll often chase off one demigod, and may only have a few seconds before backup arrives. Being able to go from no mana and half health to full mana and health before that happens is well worth the cost.
One of my favourite ways to use heart is in the middle of battle. Use a stun or activate something like a wand of speed to move out of attack range, then use the heart. Even if you don't recharge to full health and mana, you'll still get a lot of juice out of it. A close battle can often be decided if you have enough mana to use a powerful ability one extra time, and a good in-combat application of heart is the perfect way to do this. The thing recharges so fast that even if you fail it's not a big deal.