The Defensive Stance is effectively useless, and is as likely to get you killed as it is to be useful. It's been said a thousand times on this forum, but I'll say it all again just in case. It has trouble selecting targets, and with PotM's morale scripts that cause enemies to run away mid-combat, it'll break if you're targeting those. This often drops not only the stat buffs of Defensive Stance, but all queued actions and combat outside it, leaving you flat-footed. This negates the juicy sneak attack immunity. For a class to be defeated so soundly and consistently by its own abilities is frustrating. It makes you not want to use Defensive Stance at all. I seriously struggle to babysit my character through these combat breaks with 400-500ms ping, which shouldn't be necessary for a tank character - that's why I rolled a Dwarven Defender at the time, because my internet speed means I can't reliably play squishier characters.
Compared to Barbarian, I can't move around with the self-buffs from my class, I don't get that whopping 28 Spell Resistance, and I only get 1 more point of Damage Reduction. I'm not sure the AC bonus is even particularly valuable, I don't measure up any stronger defensively than pure BAB classes with Improved Expertise, I'm limited in the number of times per day I can use it, and it's a marginal difference in AB that I'm getting. I could use these feats myself (instead of Defensive Stance) but, compared to Barbarian, that would negate any strengths of the DD class over Barbarian except for the flanking immunity; that's really the only thing I'm getting outside of Defensive Stance, and Barbarian is definitely getting a lot of other buffs to make up for it. For a prestige class devoted to defense to feel such lack of strength as a defensive class is disappointing.