[Lazy_Perfectionist]

Quickness is a very iffy spell.
Don't underestimate it though, especially the remote version you get at level four research.

On a super combatant it is deadly _to_you unless you can beat twice your melee encumbrance with reinvigoration, and if you face fatigue causing or get an encumbering chest wound...

Thugs, however, can come in much greater numbers and are often well supported with regular troops. It can be pretty valuable to cast quickness on either the meat (via the level 4 research spell) or magical kind. As long as melee encumbrance is at no more than 5 (personal rule of thumb, not result of any testing) or they have reinvigoration (earth bless?), it will make a group of relatively good units a lot more effective at breaking through the enemy line and destroying their formation.

Beware of quickness if you've got other spells piling up more than 20 fatigue.. IIRC, 30 fatigue eliminates the attack bonus, 60 fatigue eliminates the defense bonus, and criticals and unconsciousness are much closer.

Highly worth casting on low-encumb, strong, glaive wielding troops (not commanders) like the sacreds of Late Era Atlantis though. They'll carve through units like the wardens of man (with a mild bless), and close with the enemy in record time, hopefully limiting your archer exposure.

With quickness, I recommend at most a light gem investment in your thugs, and sticking with meat thugs or capital troops at the most, not any powerful mages. If a mage gets afflicted, they'll be a lot less effective in combat. You can use them elsewhere, for say research, but you'll still spend a lot of gold keeping your best in the fight.

With quickened thugs, think of them as a sort of cavalry, not Heavy Infantry. They survive best by dealing as much damage as possible, taking out the opponent's best units so your regular troops don't have to face anything special. If they have a decent base weapon (9-10 damage) and strength (12-13), they can carve through armored units with surprising ease. Although you have a distinct trade off in durability, smaller is often better, as well. You'll see less of a 'multiple attack' defense penalty on your units and much more of a defense penalty on any large units you'll be fighting. Conveniently, there is that level four quickness spell that targets an entire square.