I have some insight when it comes to the bullet debate. Now, before I type this post and possibly get flamed for it, understand that what I am posting is first hand knowledge. Not some internet lore or keyboard commando puking up information he read somewhere. My opinions have been formed by experience and first hand knowledge, but again, they are my opinions!
As someone who spent quite a few years in law enforcement, 2 tours in the desert, and has also been shot himself, I have seen and felt what various bullets are capable of. In the beginning of my career (LEO), i worked 6 suicides and got to respond to a few GSW. I saw everything from the initial response to the coroner's reports with pictures. The only ones that really stood out were:
12 gauge 000 buck- male sat indian style in bed, put shotgun in mouth. Everything from ears up was gone. We were picking brain frag out of the ceiling. Absolutely devastating, gruesome wound.
.357 - male put pistol under chin in his backseat. Absolutely destroyed the cranial cavity. I've seen this same GSW with everything from 22 to 45 and nothing did as much damage as this (pistol wise). This round has consistently shown me to be devastating regardless of where it hits the body.
Now this takes me to my desert adventures, and the part that will undoubtedly chap some hides here. It didn't take long for me to develop a hatred for the 556 round, and the M4 in general. I was running convoy duty, and the guns rode in the up-armor way more than it was used. They collected un-godly amounts of dust in a very short time and were unreliable at best. I dont remember a single engagement where we didn't have at least one gun go down, (albeit temporarily). These were standard issue colts, or "milspec" as the fanboys like to call it. I was also an armorer at the time, and cannot tell you how many parts we had break in the 9 months I was there. Remember folks, these guns are not the best made, they are the cheapest made! But, I'm getting off subject.
Back to the ammo debate. The good old 556 round! This round has proven to me, and most everyone I know that actually used it, to be a joke. There are multiple accounts of the lackluster performance of this round since the wars started. I have personally seen hadjis take multiple rounds to the torso and not go down. Throw in a good cinderblock/clay/brick/wood wall, and you have even more problems. The problem we had mainly was over-penetration. I swear it was like a damn hot poker being thrown at them! Where did we come up with the idea that shooting small, under-nourished men with a high velocity 22 caliber bullet was a good idea? Oh yeah, add in the high doses of khat in them and it makes for a fun day. Upper chest/head or pelvis area was generally a takedown, anywhere else, you better shoot twice. Most of us left our M4s in the rack and grabbed the nicest, shortest AK we could find. Not only were they easier to handle in a vehicle without all the BS crap we had to attach to ours, but the it was day and night when it came to bullet it fired. I'm sure there are some vets on here that can tell you what a wound from a x39 looks like up close. It is not pretty. I can get into plenty more detail on this at a later time if you would like.
Now, onto personal defense ammo. I carried the Hornady Critical defense ammo for awhile, and tried my best to like it. I generally love Hornady's stuff, but in all of my testing, I couldn't get it to reliably expand. We shot everything from goats and hogs on my uncle's farm to water and ballistic gel, but couldn't get it to perfectly expand every time. This led me to talking with various members of my family (several retired Mil/Fed LEO). Of course the 357 came up a lot, but a few more were highly praised also. I carry 9mm or 45 exclusively so these are the caliber's we concentrated on. An older round that had a lot of good "street-cred" in 9mm was the Federal 9BPLE round. This round approached the 357 in terms of power and devastation, and as far as I know, still holds the record for greatest percentage of 1 shot stops in 9mm. Another, more modern bullet was the Winchester PDX1. This is the same basic bullet as the renowned "Black Talon" but with a less offensive name. I have intimate knowledge of this round because I have been shot by it! I wont get into the details, but I was shot with a 380 PDX1 round and I can tell you the bullet expanded perfectly and did an extreme amount of damage for such a small caliber. 2 surgeries and almost 2 years later, I still haven't totally recovered. Since then, I have extensively tested this round and it has performed perfectly every time, in all mediums! The only rounds I now carry are the PDX1 and occasionally the 9BPLE. (The wife carries a Kahr P380 with Corbon's DPX for a little extra penetration).
This has been my experience on the subject. I do not claim to be the burning bush of knowledge when it comes to guns and ammo, and I welcome debate on the subject. It has been a lifetime hobby/career of mine and I love talking about it!
It's gotten better since this time last year though. I particularly like the high velocity it provides while still maintaining decent accuracy, at least in the M&P I've shot. 
