The Mastery's have been thoughtlessly implemented without any sense of balance or logic, resulting in the usual chasm between worthwhile/not worthwhile, e.g:-
Bards: Instantly start with level 3, must remain in range, and can be completely stopped by damage.
Tamers: Level 3 legitimately costs 40m+, why? Because at level 3 you get a non interruptible, non range checked, 4 mana per second invincible pet. (apparently the Devs thought playing a Tamer was too involved/much effort)
Spellweaving: Level 3 legitimately costs 30m+, why? Because at level 3 you get a Summon that also casts WOD, and Mana Shield which is just fantastic.
Necro and Ninja ones are pretty good but not worth much, and everything else is pretty much worthless, a few minor boons like (apparently) less chance to be Disarmed, 15 Mana etc, but nothing like invincible afk mode like Tamers.