Character with 8 dex can easly hit 18 dex (6 item +2 tome +2 guild). It is +4 dex ac. Adamantine body gives 8 ar + 1 max dex bonus, mitrhal body 5 + 5 max dex bonus (can be more with mithral fluidity). Conclusion: character with 8 dex can have same ac in both adamantie and mithral body. In adamantine it lose evasion, -5 armor check penalty, 35% arcane spell failture and it gives 2 adamamtine DR (marginal) and character in adamantine body will lose less ac from dex when stuned,paralazed but WF is immune to paralize so its marginal too. In my opinion this feat is usless. Improve it a bit? Set 25% spell failutere? Anything?