I bought the heads, their prostar, I have looked ever them very well everything looks good, the machinest i bought them from assebles them with his own things, he buys them bare and puts guides in etc.. The casting is great, I thought they where a great buy for $750.
If you read through enough of the google searches you will come across a lot of negative comments about the procomp heads. Most of them that you read will be like the ones earlier in this thread. Folks who have never had them but heard stuff and say they should be avoided. Or they searched google and saw bad comments posted in forums from guys who blast them and have never owned them. Lots of that "a friend of mine" stuff without first person feedback. Read through this brief thread link (end of post) at a forum asking the same thing. What you will see is only three of the responses say they have used them and none had problems. One does say he will be switching them from his tow rig but no "why". Of the negative posts, none claim to have had them and had a problem. Not a single post from a person who said "mine were junk and didn't work". I bought a set and have them on my 408 build. I have not driven it enough to know how well the work or discover any failure modes. I will get the car on a chassis dyno some time late this spring. But I did a ton of research before I bought them and actual reported failures are pretty slim. The ones I did read did not seem out of line than problems reported with other heads. And some of those failures I read about were dubious in nature. http://www.moddedmustangs.com/forums/5-0-mustangs/139095-pro-comp-cylinder-heads.html