Pandemic and Forbidden Island - How similar are they?
While I have both and definitely notice the similarities, I don't regret the purchase.  Here's why:

1)  Forbidden Island is easier to teach.  Not that I think Pandemic is very complicated, but I've been able to teach Forbidden Island to my 8 year old where Pandemic was a bit too much.   Also, the fewer number of bits (cubes and such) has made the game less intimidating to spouses.

2)  Forbidden island is easier to vary.  Beyond raising the "damage coefficient" (number of infections in Pandemic, beginning water level in Forbidden Island), you can alter the tile layout to form a new challenge.

3)  Forbidden Island plays quicker.  Faster setup, faster game play, faster tear down.

Having said that, my usual gaming group prefers Pandemic with expansions to Forbidden Island because they feel it is a deeper experience.

So to sum up:  I would only get Forbidden Island if difficulty, approachability, or time is keeping Pandemic from hitting the table more.  Otherwise, I'd go for another coop game such as Ghost Stories or Castle RavenCroft.