There are a lot of misconceptions about Krav Maga. A lot of people who post about it have never done it and don't even know what it consists of. I've done it for a few years (along with BJJ), so I may be able to clarify a couple of things.
First, if you have any interest in this topic, it would make sense to at least take a few seconds to look at the standard KM curriculum, which you can see here:
Krav Maga DFW Curriculum Main Page
The curriculum consists, for the most part, of very basic strikes and defensive techniques taken from other martial arts. The goal is to fend off a sudden aggressive attack as quickly and efficiently as possible and get out of there. If someone becomes proficient at these techniques, I think it could serve them very well in a self-defense situation. And herein lies the problem. Like with any other martial art, it is not easy to become proficient at these techniques. It takes years of good, hard training.
A very reasonable criticism of Krav is that there are likely many schools out there that have poorly skilled instructors who give their students a false sense of security. There are students who train one or two hours a week for a few months, skip sparring classes (if they're offered), and think they'd be able to defend themselves. On the other hand, if you have skilled instructors and train hard over an extended period of time, I think these techniques can be very effective.
My progression has been the same as it has with other martial arts (like BJJ)--after a few months of training, I think I'm getting pretty good; after one or two years, I realize how much I suck, and the real training begins.
On a final note, people frequently criticize Krav, saying you can't test the techniques in class at full speed in realistic situations. This is hard to address because, it's true, you can't elbow your training partner in the face full force, or kick them in the groin if they're not wearing a cup. Where I train, we do the best we can by doing some boxing/kickboxing sparring (with gloves, headgear, etc.) to practice movement, hitting moving targets and feeling what it's like to be hit, and by doing some of the Krav drills with attackers wearing protective suits so you can defend at full force.
If anyone has questions about Krav training, I'd be glad to try to answer them.