It's not bad to tide you over, but you will soon find better ones to replace it. If you are main spec protection and offspec retribution, I'd advise picking instead the strength one. Strength is better for ret than agility is prot and as a main spec proc, you'll find it harder to roll on ret trinkets.
However, if you have already picked it, it is ok. Agility translates into about 0.6 dodge. The proc is a lot of mastery, but with mastery I prefer a constant amount - as it seems to me all about smoothing damage - or at least a controllable on use proc. Tia's grace - a drop from Lost City - is better mastery/agility trinket - constant mastery and, effectively, constant agility. One of the Tol Barad trinkets gives you constant mastery and a on use amount of agility. Impetuous query, available from the AH, is also better iirc.