How effective are elbow blocks?

You said that this type of "wrapping the head" elbow block is used in various martial arts and MMA, but I have personally only encountered the strike in an MMA setting.  (The coaches claimed that it was also used in kickboxing.)  I am not a big guy.  In fact, I was the smallest man in the club, and it was sometimes difficult to find training partners of my own size.  In working with people bigger than myself, I too noticed the amount of force that still transferred into my head from strikes.  The reality is, though, that in the ring, I would be matched up with someone of my same weight.  That is the limitation of this block, as far as I can tell.  It is a tactical block, designed to allow us to absorb some impact from a few strikes, before we retaliate with a combination of our own.

After my brief stint with MMA, I returned to karate to find that, although it was not my choice block, I would occassionally wrap my head, in a situation where my partner advanced too suddenly, and I needed a quick defensive technique.  Again, I am not a big person, and I have found that it is much easier to stay safe when I focus on intercepting my partner's movements and moving evasively (and stratigically). 

To further justify my position on this block, I encountered it in krav maga as well, in scenarios where the opponent has mounted you and is raining down blow after blow after blow.  As you work to position yourself so you can flip your attacker over, you wrap your head with your arms and essentially ride the strikes -- trying to stay concious.  Again the block serves as a desperate measure in a bad situation.

I'd conclude that the block, as it is presented in MMA, is designed to defend against opponents of similar sizes in a competitive setting.  When used in a self-defense context, it tends to show up as a way to control damages when you're defensive options are very limited. 

I believe that I've also seen it in Fillipino systems where it's used in conjunction with a slip and the other hand is used to guide his striking hand onto the end of your elbow. Supposedly to break the fist. 

Maybe It depends on your structure and the  direction of the attack vs. your body if you are using it to stuff an attack it can be quite effective...when you try to just use it as a shield though, it doesn't absrob so much. I actually wonder if you could conceviably get knocked out/down  through it - as I know like you say I have definitely felt some impact into the head and neck just using it as a block.

It's worth noting this is seriously common in Goju Ryu kata, it's like half of em in one form or another, arguably more. I think the effectiveness of it is about timing and structure, anything done as a dsicrete 'defense' by nature isn't really ideal, and if we frame the technique as a 'block', that's basically what we are doing with it - right?

I really like the use of elbow blocks, and covering using the arms. We have a 'Duck & Cover' curriculum that teaches this as a standalone defence posture. This is taught as a flinch response as it seems very natural for people to level change slightly and grab their head in the face of unknown contact. 

 When combined with the Elbow strikes from Shotokan we have been using the term, 'Monkey's Elbow' from one of the old Funakoshi books to represent our training with all the traditional elbow strikes and defences using similar motions. There are some subtleties to the techniques however; you need to build proper frame theory and understand how each arm position works to protect your head. None of this goes on without movement and footwork, so if you’re getting hit hard through the defence, you probably need to move more.  

I have seen this type of defence work in DT situations from officers, to MMA fights in UFC, to boxing matches. It must be noted though, that this defence works in conjunction with forward pressure to close the distance and clinch. For the officer, going in for an arrest takedown, for the boxer going in to fight on the inside, for MMA going to clinch for attached striking or takedown. It is not effective to stand there and block strike after strike in this manner. Also it is only a stand-up defence, as using your arms this way on the ground is asking for one or both of them to get broken for you (sets-up the armbar), but it does restrict the opponents ability to grapple with you when you remain standing as it takes away the natural hand holds for grabbing and holding. 

I'd be interested to hear how it is working out for you, and in what kind of drills you guys are experimenting with it. I believe that it is an important and funadmental movement and concept, and as such I teach it almost right away to beginner students.

Blocking a blow is never as good as deflecting it, wich is never as good as avoiding it. When you put your hand there it is still absorbing the shock, so you should expect some impact. The important thing is, its not all of the force, and its not straight into your head. You should always try to avoid being hit, but see this as a last resort. There was an interesting thread I read on another forum about this point- the reliance on blocking or keeping your hands up. It was a story about a pro boxer that always kept his hands up, and just presumed everything was fine beacuse his arms were absorbing the shocks. Sadly because he relied on that so much the force went through his arms into his head, like you described, and he ins now suffering very badly from pugilistic dementure.

That is not to say blocking punches is useless, but you should not rely on it solely to protect you and expect everything to be ok.