I'd say krav maga is pretty good for someone who wants to learn how to defend themself. It should work against an inexperienced fighter, a streetfighter, and even someone with some martial arts experience as long as you are well practiced.
In krav maga you try to end the fight quickly (which is good in a street fight or in any situation where your opponents friends might get involved). It is a more offensive style. When someone attacks you, you counterattack simultaneously.
Since MMA has become so popular alot of people criticize krav maga and say that any good MMA fighter could beat a krav maga practitioner. I would answer: do you plan on fighting a professional fighter in the street? I don't...
Krav Maga is used by FBI, CIA, DEA, SWAT teams, and Isreal's military (I'm sure I'm missing some). It's effective in real world situations and they teach you how to handle an opponent who comes at you with a gun or knife or other weapon.
If you want to learn a style in preparation for further mma training maybe something else would be better for you, but krav maga has grown over the years to incorporate the most efficient and devastating techniques from boxing, wrestling, jui jitsu and other martial arts. These techniques are often not legal in sport mma because they can cause serious damage and death.