Pandemic - Will the expansions really make a difference?
First make sure you are playing the game correctly. From BoardGameGeek FAQ:


  Q: Our first game was easy! Are we doing something wrong?
  
  A: Probably. But the game is easier with fewer players and/or fewer Epidemics in
  the deck. You may also visit this thread to see if you missed
  something: Common mistakes.


Also try the game with different number of players. A two-player game is different and apparently easier than a four-player game. There is also quite a big lucky factor involved depending on how the infections are distributed and how early the epidemic cards occur.

If you still find the game too easy but otherwise like it, consider buying On the Brink expansion. It adds variety to the game by providing a lot more roles. It can also make the game harder by offering new challenges.

If you don't like the game, you probably don't like it with the expansions either. For me the base version was fun on it's own and hard enough, as we failed to find cures for any deseases on the first game with the easiest difficulty level when playing with four people.