Jacky thoughts; after over 200 games and a less than 40% win rate:
6P variants - Jacky's bread and butter poke. It's neutral on hit and block, a natural combo with 6PK or 6PP and it has a number of variants that hit at all heights (6PPK: Mid, High, High; 6PPP: Mid, High, Mid; 6PP2K: Mid, High, Low). The 6PP is a natural combo that's safe on block and -4 on hit, it contains and unless the player is getting repetitive, there's no real way to predict the final hit's height. All three finishers stun two of which aren't SE-able. 6P is fast at 12 frames making it on par with the Ninja's and other striker's 6Ps. Although I feel that Jacky's may be on the lower priority side akin to Kokoro's 6P.
Jacky's Auto-Punch Parry - Catches all high punches and slaps the opponent for a small amount of damage and puts both players at neutral. Useful for mind games as the opponent has to learn that high punches are automatically held meaning that Jacky can contain and force players to use high kicks or are stuck with a mid-low game which makes it easier for Jacky to "guess" hold mid or low attacks; as far as mids are concerned this works out to his advantage thanks to his mid punch hold.
4PK - A high, high knockdown move. Not very useful as the damage is pretty minor and its not a natural combo so it doesn't lock the opponent to blocking both hits. If they do, however, Jacky recovers back-turned with +3 frame advantage. Kinda useful.
64T - Jacky's strongest throw at 66 damage (better than Kokoro's three part throw by 1 point) can reverse positions depending on stance. Has monstrous throw range, Bass would be jealous. The nice thing about this throw is the huge damage off counter hits, easily hitting 100 damage. It's also one of the best options post mid-hold for Jacky as it puts the opponent right in Jacky's throw range. 12 frames.
46T - Jacky's other major command throw. Second strongest throw at 60 damage, but has a unique mechanic in causing the VF5-style wall bounce when near a wall. Although you can get the wall bounce, you can't quite get the damage, but there's a number of combos, including wall-slams and cliffhangers that can follow after this throw, bring its potential damage up even higher. Highest I managed on a basic wall slam combo is 77... maybe you guys can find better?
Jacky's mid hold - Jacky's mid hold looks pretty weak despite its long animation, the beauty is after the animation Jacky is left with a massive +10 frame advantage to literally do whatever he wants. If the opponent sits blocking, you throw, if he sidesteps, throw him! If the opponent attempts to jab, Jacky 6Ps. Not even Pai gets it this good.
66K+H4 - Jacky's CB with an optional 4 input to leave him back turned. No matter the distance, this always leaves Jacky safe at -3 and the BT opens up some shenanigans. Kinda risky, but the rewards are pretty good.
3P+KT - Normally Jacky's combos start with 4H+K or 23P, once you put them in stun, Jacky can whip this out for some easy damage and a "Come on!" taunt just to rub it in.
[6]K:H - Jacky's just frame knee. Perhaps the sole reason why Jacky moved from beginner to intermediate - advanced players recommend. Although not nearly as difficult to pull off as Akira's, its usage is more pronounced in both VF5 and DOA5U. The move acts as a re-launcher for many of Jacky's major combos. Slam an opponent for 4H+K, then [6]K:H and repeat two or three times to eek out a bit more damage. The move is best known as the Flash Sword Kick, or FSK. Its not vital for successful Jacky play, but it does give a lot more combo damage. Don't bother using this online as the lag will screw up your timing too much.
3KP2KK - A rather famous Jacky string in VF5. It has some pluses in 5U too. The 2K hits low and gives a +4, the last kick is guaranteed if the 2K hits and 3K has quite some reach. Because the move is kinda famous, everyone pretty much knows the pattern and can block all of it. So its not that useful. The variant, 3KPK isn't much better since the last hit is high and can be ducked mid-string during block. With that said, 3K is actually pretty good at +5 on hit and thanks to the range is can work as a solid interrupt.
PP variants - PP is a natural combo, P by itself is +1 on hit and -1 on block. The variants continue as either highs or mids, but some of them have built in natural combos too. One example is PP6PP, the 6PP follow-up is natural on the second P, meaning if they don't hold 6P then the second P is guaranteed and then this leads to three follow-ups a high kick, a mid punch or a low kick. PPP is all high with the last hit being a knockdown, damage is low, however, and this string operates very similar to Kokoro's PPP string with the last hit being duckable on hit or block, but it is guaranteed on counter hit. PP8P+K is a sidestep into a stance and why you would use this I have no earthly clue. PPK is all high with the last hit being a lengthy stun that can't be slow-escaped (or held for some time). PP4PK is also all high, the 4P stuns on normal hit. For the most part, Jacky's PP string is less varied than Hitomi's and probably less rewarding. This is not to say that Hitomi actually has better follow ups after her PP, though...
3PP - This is a natural combo that stuns on the first hit. Can be useful to extend stun time too. Both hits are high and a high follow up (P) and two mid follow ups (P+K and [P+K]) the second mid follow up has a useful guard break if fully charged while both mid hits will bring the opponent into stun threshold for 66K+H CB, though only one of them is slow escapeable.
1P+KP variants - Mostly used as combo filler. Despite the animations, Jacky isn't in crouching when using this string. For some reason, a lot of players eat the P, seems to have a lot of priority.
1H+K - Jacky's sweeping low kick. Unlike VF5, this move has a positive frame advantage on hit, knockdown on counter hit. Kinda useful.
2H+KKK - Jacky's lightning kicks. At 12 frames, very fast and pretty strong. Block frames are deceptive. The first three hits are, yet again, a natural combo and can't be held. Mix up one, two, or three hits for fun and profit.
KPPP - This is a natural combo. Get hit by the first K and opponents stuck until the last P. Further, at the end of the string opponents are at red stun and can be CB'ed. The good news is that the stun is SE-able making this string kinda gimmicky. Luckily the finishing K can be delayed or not used at all. On counter hit the string makes it difficult to hold after the last P.
Slide Shuffle Step (44P+K) - I have no idea what to do with this.
Fire Darts (Running K+H) - Yeah!
Auto-Taunt - Jacky automatically taunts after certain moves and strings. Some require hits and others don't. I suggest trying out a lot of the moves that have taunts in them just to piss off your opponent. A lot of Japanese players who play Jacky say the taunts are useful in flustering your opponent because many of the taunts are tied to some of Jacky's worst strings, meaning if they got hit by it, they got hit by a bad move. How embarrassing.