I have actually had the experience of having a player run an iron half-golem from low levels all the way to level 18 using a homebrewed monster class progression. It was quite the experience. I set the LA of the iron half-golem template at +8. My impression of the overall experience is that it wasn't enough. I now think that perhaps +10 would be a better level adjustment, maybe even higher.
It cannot be overlooked how well the abilities of this template synergize with itself. Massive bonuses to Armor Class, Damage reduction, and total immunity to magic had this player virtually untouchable by high level. The thing about natural armor increases are that there is nothing that a character can lose by giving up class levels that will bring their AC into a normal range because most classes don't gain AC from class levels (unless your players intends to run a half-golem monk). This means that no matter how high the LA by the end of the progression this character will be almost impossible to hit. High damage reduction means even the hits that do manage to land seldom do any damage. Magic Immunity rounds the picture out by making the player immune to (or even healed from) almost all magic.
About the only thing I found effective against the player were high level giants wielding adamantine weapons, and advanced hit dice versions of monsters that had such high attack bonuses they could power attack and still hit him. Originally I thought that the loss of offensive ability would compensate, but the high strength bonus meant his attack bonus was almost as high as it would be had he taken class levels and earned a better base attack bonus. He did get fewer secondary attacks, but often those miss anyhow, and the player made up for it by being able to power attack for more on the attacks he did receive.
Basically, if you do allow a player to run a half-golem, I know the Iron golem version is worth around a +10 LA. At any rate, just keep some antacid tablets handy and good luck to you.
