Bring on the Cavalry, Light cavalry I mean

[Hold Steady]

HOBILARS ARE MAGNIFICENT! Since you ask, I agree wholeheartily! If I can get my hands on any (early) cavalry I'll do it, since Hobilars or mtd Sarges are perfectly usable for flanking, harassing, in fact, for anything bút head on charging into any kind of infantry that can put a dent in a horse unit. Hobilars and such that can crash into the rear of say, men at arms, (while this unit is pinned to any kind of inf) will severely damage the unit and if you keep a bit of an eye on it, one hobilar unit will turn the tide of the battle if you let it hammer on the back of engaged troops.
The thing is, light cav (incl steppe and such) will be chewed up and routed if they attack a decend unit head on or when mired down in a general melee. And, they get freaky if flanked and threatened. The heavier the cav, the meaner it gets on head-on charges. And I'll be weary to engage spearmen from behind with light cav. I won't be with heavy cav..

Thing is, if you want something to take down archers, very light infantry or non-spear/halb infantry from behind (in accordance with pinning from front, or chase Horse archers, take light. If you want to do anything else, take at least medium. If you want a tank to drive through an enemy front, take the heaviest of the heaviest (Gothic, Lancers, Crusaders, Late royals).
I must say, If I want to stop a charge of an enemy Heavy cav, I would rather try to take 'em head on with spear units and flank them with medium cav, than take 'em head on with heavy cav..

[Procrustes]

Hobilars are great in early because their build requirements are so low, but they are prone to route - I stop building them once I can get mounted sgts. I pile the armor upgrades on my mounted sgts, too - it slows them down, but I figure it keeps them in the field a little longer.

Personally I'm not a fan of cav heavy armies - I tend to have one or two knights, two light cav and/or two mounted xbows/archers - the rest a mix of bows and infantry (which should include spears and some polearms and/or axes.)

[Ludens]

I pile the armor upgrades on my mounted sgts, too - it slows them down, but I figure it keeps them in the field a little longer.

Armour does not slow your units down directly, though it does make them tire faster in the desert or when it snows.

I like light cavalry as well, especially mounted sergeants, but I prefer horse archers. They are faster and can really disrupt your opponent. Of course, you need other units to exploit the disruption since many horse archers are weak at melee combat, but nothing is so satisfying as turning a well-organised enemy army into two or three disorderly masses and then charging in with heavy cavalry.