This is the most effective DD spell we have in our arsenal, hands down. It not only gives damage that is equal to a meleeing hit (more at later ranks), but also gives DoT that basically equalls half a melee hit every 3 seconds. While casting this spell may not give you big, fat numbers, it will do much more in the long run than Starfire or Wrath will do.
In case someone had not noticed, during battle we have no time to be wating for a spell to be cast...and our spells are considerably long cast times. We are standing there meleeing, no time to wait upwards of 8 seconds for starfire to be cast, since by the time it's off we could have done the same ammount of damage with our staves (or whatever the crap you use). It ends up being just a waste of mana. Never cast Wrath or Starfire in battle if you are meleeing...never.
With that said...enter Moonfire. Moonfire does basically the equivalent amount of damage as Starfire except it is over time...making it seem like less. Think of it this way. Rather than stand there getting hella-interupted trying to cast Starfire, you can cast Moonfire INSTANTLY and have Starfire be cast slowly while you melee! As soon as it runs out...BOOM! Moonfire again! You can usually get off two each battle, which equalls a lot of damage that you didnt even have to stand there casting. This spell compliments soloing/meleeing PERFECTLY.
You will find yourself casting Moonfire all the time: Pulling, nuking during battle, finishing enemies off as they run away...endless uses. Even when you go for that big, fat mob that you are scared of. That one that you entangle to bring his health down. The one that you COULD cast starfire or wrath on since there would be no interruption. You will still cast Moonfire...I guarantee it.
As druids we are blessed to have such a powerful instant-cast DoT nuking spell at our disposal. It compliments our meleeing perfectly, and is probably our biggest mana drain.
For all you druids out there, I strongly urge you to follow the Balance talent tree. Its benefits to Moonfire are wonderful (+20% to damage, +20 yard range, -10% mana cost, and +critical hits if planned right) along with some other nice talents that also benefit you (clearcasting state, no downtime for casts between animal and druid forms, natures grasp). I know that picking a talent tree is one of the hardest things to do for a druid, since we are supposed to be good at everything. However, the last thing you want to do is give up moonfire.
Think to yourself how often you use this spell. Learn why you love it...BOOM!