Personally, I think that something like this is a much-needed patch for the system at high levels. Right now, high-level characters can only fail against a lot of abilities on a natural 1, but they're guaranteed to roll a natural 1 on a saving throw sometime. This means that a pack of beholders is not much more of a threat to the party than a flock of cockatrices.
As the Epic Level Handbook itself acknowledges, this is especially a problem with death by massive damage, once you reach the point where every blow does at least 50 hp. No matter what you do to raise your Fortitude save, you still die on a roll of natural 1, and you still have to roll several of those a round. Steadfast Determination means that all those dinky little DC 15 Fort saves can no longer kill you, so your survival is no longer a matter of pure luck. Tanking becomes possible again.
Indomitable Soul doesn't seem as if it fundamentally changes the fact that enchantment spells are a huge gamble. Succeed, and you're better off than if you'd killed the foe, but fail and you've wasted a spell. Many creatures are totally immune, and there are some very effective wards. The size of the bonuses don't seem excessive, and the need to roll snake eyes on 2d20 to fail a piddly low-DC save is a good, and much-needed, counter to what would otherwise be the cheesy tactic of using Automatic Quicken Spell + Multispell (or other similar combinations) to make opponents roll again and again and again until the die comes up a 1.