Is the Melon-Pult worth its price?

Melon-Pults are the best if you use them right, especially in endless levels. They are best used on the lines just below the top one or just above the bottom one due to their Area Of Effect.

The DPS you get from a single plant is much better then any other plant you can get due to the Enemy clumping up in large waves. Let's say you got a tall-nut with 5 zombies chewing on it. All 5 get hit at the same time by a Melon but the repeater only hits one.

And to top the Melons off, if you upgrade them with frost bonus you can effectively slow all incoming enemies with just 2 plants total. So, Yes! The splash is worth it, and the slow bonus you get with upgrading it makes it even better!