One of the campaigns I DM for is based in Greyhawk and is using Liberation of Geoff as a basis for their adventures. Obviously, a lot of the opponents turn out to be giants. So far their encounters have only been against HIll, Frost and Fire giants, so my experience with how they've fared is limited to those 3 types facing a 6 character party across levels 7, 8 and now 9.
Based on the results of several battles, hill giants seem to be about a CR 7.5. Against a 7th level party, they were a tough match, sometimes acting more like a CR8 or 9. But in other battles, they've gone down very quickly with barely a scratch acting more like a CR7 monster. Now with the party at 9th level, they are pretty easy to take out without much danger. The hill giants' achilles heel has been no so much their will saves, but their reflex saves. Spells like entangle can be quite useful in slowing them down, providing the party with a better chance to fight at range where they usually have a sizeable advantage. And they tend to suffer full damage from most area effect spells as well.
The Frost Giant is listed as a CR9, but in battles so far, it's often seemed weaker than that. They do have a slightly improved AC and better attacks than the hill giant. But in game play, they don't seem much better than a hill giant. Once again, they have a pathetically low reflex save which usually means they take 150% damage from every fireball sent their way. I think they behave more like a CR8, at least as far as this group is concerned.
Fire giant is listed as a CR10, but these seem more like a CR11 or 12 for my party. They have even better AC, better saves, greatly improved attacks (3 swings with full attack and higher attack bonus), and of course that immunity to fire. While they do take 50% extra from cold, the most typical choices of cold-damage seem less reliable (ice storm, cone of cold). As a result, melee combat nearly always occurs with them. With 3 attacks that tend to connect fairly often for 3d6+15 each, a fire giant can quickly drop even a well-armored character in a very short time.