I'd say there is a portion of hard work and a portion of luck involved in BUD/S. Some guys are hardcore, and even though it hurts them they make it through and you never hear them talk about quiting, etc. Some guys are hardcore, in good shape, have excellent minds, and just get the raw end of the stick by getting hurt somehow.
It is definitely hard on the mind though. Have any of you quit smoking? You know how after a little time you start convincing yourself that you dont really care about smoking and that its okay to have one? Well BUD/S is the same way, its easy to let your mind get carried away and convince you that you dont really want to be there or that you just want to plain do something else with your life.
I dont necessarily agree with all the of training. I do think its a kick in the balls, but I dont think that you make it through the final stage with someone that is any better than guys that made it through other special programs. I have as much respect for Green Berets, Force Recon, etc. Perhaps more respect for Delta and probably covert CIA because in many ways it is superior intellectually.
But yeah, guys that make it through BUD/S definitely get props, because no matter who you are it is a kick in the balls. As to ratios of guys that make it through, its pretty low. Officers have a much better chance of making it through, though. Not because they are treated any nicer, but because they usually have a bit more fortitude.
I'd definitely recommend anybody that wants to give it a try to go ahead. The downside is that if you end up not making it you'll either get shipped to fleet or put in 9545 (not sure what MAA retention is these days, perhaps they are just sending to fleet now).
For people that want some of the fun and less of the kick in the balls, I'd recommend SWCC. Its a hell of a lot easier physically and mentally, the training is alot shorter, and its a pretty nice job from what I've heard.