I will not sit here and boast confidently that I am fully prepared for a real-life fight and will kick the other guy's or guys' ass(es) or whatnot, but I will say taking up MMA has given me some real, valuable skills that I can use as well as helped me in my confidence a lot in case something like that came up. I definitely believe I need more training (I guess you're never done really but you know what I mean -- iron out the basics and all that) but even having a little training is better than none.
Some schools don't teach you things you need to know. In a lot of martial arts schools, the reason the martial art seems to be "useless" in a real-life fight is because the school teaches it based on TOURNAMENT rules. For example, in taekwondo, you're not allowed to punch to the head. So guess what? You're not gonna learn a whole lot about head shots and blocks at a lot of taekwondo schools. For an even more obvious example, in a lot of disciplines if both fighters fall or go to the ground, such as in karate, the fight is returned to stand-up. In real fights, there IS head punches and ground fighting.
Even with all these cool things I'm learning, I know real-life fighting is different than sparring. There's concrete and asphalt, not mats or boards (like the floor of rings), guys can pull weapons, there can be multiple attackers, people are allowed to bite and eye gouge, etc., so I can't do EVERYTHING the same as I would in training and I might have to think quickly to react to certain situations that can't really be simulated inside the school. However, having this kind of training definitely doesn't hurt. When you fight full-contact, minimum-rules (like UFC fighting), even if it's just sparring, you still can get used to being hit hard on most parts of the body and get the general experience of some guy trying to "kill" you. At the very least, the guy may have more street experience than me but generally speaking brawlers lose out to guys who have real, hardcore training in all aspects of fighting, including beating hit, which if you're not used to can really cause you to get panicked. If he tackles me and gets me to the ground, I'll at least be able to get out of the bottom and on top, and I'll at least be able to know how to give a good roundhouse to his thigh if we're standing up. An MMA school may not simulate a REAL, real-life fight where there's no referee stopping the fight and where's there's absolutely NO rules, but it is pretty damn close to real fighting, as close as you can get at least.
People who say martial arts don't work just haven't seen someone with the right kind of training. You think a street brawler's gonna know how to defend against a muay thai clinch? Think again. He'll be eating knees.