I have been Javalry since just after chadz, in his wisdom, paired with Jest to make an NA cRPG server. I've seen it go from just myself, to a large contingent, to nothing, and now I see it on the rise. This is good!
If you want a javalry build I can share mine with you. If you are planning on making your own, remember that it is really, really difficult to find and pick up spent shots, especially when being circled by lance cav.
My build :
Level 30 (4 420 577 xp)
Strength: 15
Agility: 18
Hit points: 56
Attributes to skills: 2
Ironflesh: 3
Power Strike: 5
Shield: 4
Athletics: 4
Riding: 5
Horse Archery: 3
Power Throw: 5
Weapon Master: 6
One Handed: 110
Throwing: 126
Tips :
Don't bring any thing that is not sheathable, including a lance. Javelins(or w/e) are meant to be used before melee, and you will have to drop your lance if you see a light cavalry offer a tempting shot, or see a shield wall you can pass behind to break up with your javelins.
Carry a melee weapon. While you MAY be able to find one when you are out of ammo, you have to slow down quite a bit to pick things up off the ground, which can get you killed (usually by ranged).
A shield is a HUGE help to javalry. If you get caught by melee cav that is on a faster horse (or many), you can use your shield to catch their lance, then toss a javelin into their face. It can also save your life when you don't have time to hit x and try to downblock with your throwing weapon. It can also save your bacon vs a horse archer, though they will usually just aim for your horse.
Take advantage of the fact that Javalry can throw a bit to the RIGHT of their horse. You can often catch a piker or lancer with this move. Act like you are charging them, then peel off a bit to the left, toss, and keep riding.