Well I can only speak about lv60 cap Ballista's so...
An important tip: TP IS YOUR BEST FRIEND. Whoever can get full TP first and then faster wins the match. 100 TP = 300-700 dmg. If someone decides to run up to you and duel, whoever gets 100 TP first will send the other running (most of the time).
What makes Ballista fun for me is the complexity (aka CHAOS). With the Diagas and Stonega's flying around killing shadows, /sprinting BLMs firing off Blizzaga IIs into the alliance, RNG shadowbinding, barraging, sidewinder-ing in 10 seconds to knock you flat, or a MNK dishing out Raging Fists, its hard to not jump in and just kill. This is the only PvP you'll get, ladies and gents. A nice hate outlet... ^^
BLM, RNG, RDM, and SAM definitely should not miss out on a 60-cap ballista. Every job has its benefits.
BLM have access to Blizga2, which can deal 300-400 dmg to 6 ppl. Solo capacity is awesome (stun/sleep, bind, Freeze) and they make great team players by knocking down all shadows with their wide array of -ga spells.
As stated, RNG are renowned for instant damage and many times, kills. A well-timed, full hit Barrage followed by a Sidewinder can kill any melee. If you have a good team, you can rely on -ga spells to take out annoying shadows and can ranged attack while melees fight.
RDM are still the jack of all trades in a ballista. Sub NIN and 1v1 is a breeze. Sub BLM for extra nuking power to finish off a fleeing player or to stick every debuff. A true test to see who has their Enhancing leveled for Stoneskin and Enfeebling for paralyze/bind/slow. Chainspell is deadly.
TP decides the match. And SAM's are masters of TP. Unfortunately, Tachi: Yukikaze is rather weak at times (200-400) but landing 2-3 in the time it takes your target to get 1 WS is priceless.
If there's any job I missed you think should get credit, let me know.
So, come play! There's a few expert Japanese players who dominate 60-cap ballistas that you can watch and learn from. 450 exp, a chance at a spiffy title, and the satisfaction of knowing you can solo a <job you hate> is nice.
Sorry if I got a off-topic, but I'd just love to see more NA players in ballista.