For the uninitiated, let me say that Plants vs. Zombies is an amazing game. It's fun/funny, and something that can be played in short bursts, or for hours (if you lose track of time...). The general idea is that of a tower defense strategy game. You gather sunshine so you can grow plants that have either offensive or defensive purposes, so you can protect yourself from the onslought of zombies. It's real-time, meaning that as you are planting the zombies are attacking.
Really the question here shouldn't be, "should I buy Plants vs Zombies?". You should, but do you want this particular package? On the xbox 360 they have Plants vs. Zombies available as a downloadable arcade game for 1200 Microsoft Points (the equivalent of $15). It's the same game as what's offered on this disc. However, this disc includes the additional games "Zuma" and the amazing "Peggle" (a $5 and $10 value, respectively). Definately something to consider if you haven't already purchased either of those games.
My one caveat would be that you can't install the games individually to your xbox, nor can you install the entire disc to the hard drive (this allows for shorter load times and for the system to run quietly. If you were to download the game it would automatically work this way). However, this has nothing to do with the actual game, just the package itself.