Machinist is a pure damage, non utility, gun wielding ranged job in FFXIV. It shines through its high damage capabilities and versatility. The strongest point of Machinist is that it does not rely on other people to perform well. Here you can find the official jobguide from Square-Enix explaining all skills.
To get a glimpse of how Machinist works you can either refer to the inforgraph on the left or the video below if you prefer that.
Machinist generally prefers critical hit just like every job in the game! Due to Reassemble, determination becomes stronger and is a valid choice. Generally you can go by CRIT > DET = DHIT. The Best in Slot for patch 5.2 can be found here.
If you want to have more in depth explanations, the 2 videos below cover all information for a beginner & for experienced players wanting to get more into Machinist. A playlist with more videos can be found here.
The epic of Alexander is our current latest Ultimate Raid and is still being progged by a lot of people. If you decide to play Machinist in TEA you can find some very useful information on how to push your Damage as much as possible with Dook's Guidelines for Machinist Tea.
Remember that it is very important to wear ilvl 470 gear in TEA as it is synched to 475. As long as there is not 475 gear in the game, the BiS from Eden's Gate (Savage) will remain the BiS for TEA. You can find it here. The amount of damage lost from using ilvl 500 gear is around 150-300 dps estimated depending on jobs.