I have a white mage which uses Staff, out of 3 Deathblow Skills, I found myself using the first one all the time (+BP , +HP , +MP) . Now the 2nd Deathblow Skills have the +5 turns , but I never understand the mechanic of these so called "+5 turns" , it seems that it did absolutely nothing :(
and while we are talking about turn, Haste seems to ONLY make it that you move first at any given turn, it DOES NOT give you any extra turn :( (the usual Hastega + Slowga on FF which would caused you be able to move multiple time before your enemy can even do anything)
I didn't rely much on the Deathblows since I never did get enough hours in for the village (I would say forever alone, but it's because I simply didn't get enough people on my friend roster at the time I was playing the game to speed things along) to purchase upgrades, so I can't help with that.
As for how Haste functions in the game, yes, it really just pushes you to go before the opponent. Given how the BP system works, it'd probably be more "fair" that way than to have it work as it does in a FF installment. Even then, you'll find once you get the Superstar class, the Time Mage class really isn't that useful.
PSN: Neophoton | NA 3DS: 2277-6964-8962 | JP 3DS: 2509-0978-4639 Currently playing: Fire Emblem: Awakening, Tomb Raider