In my opinion, Ranger is definitely a good buy. Even better if you can find a bundle pack for it. Amongst all the Pokemon games, Ranger is the most challenging, there's probably an infinite number of ways you can approach catching the Pokemon. The limitations aren't as strict, unlike the previous games where turn-by-turn basis rules are imposed(With the exception of trozei and pinball). Also, it doesn't mean that you can thrash the entire game just by leveling up your team to insane levels.
The downside is the game is pretty short, It took me about two days to complete everything, including the ranger browser. It's addictive while you haven't finished the game, but once you're done with it, the lasting value is gone.
I don't think obtaining Manaphy is the main point of getting a game. One Pokemon isn't really worth 59 SGD(The price of the game. Around US $36). Even worse, you'll proably need an event to get it.