I spend the majority of my time gathering and crafting, it's definitely a viable way to play. What I did was focused on one combat class and then the rest of my time was crafting.
It's an extremely good idea to level all crafters to 15 before taking one to 50. You get some super valuable cross class skills at 15 that you will use forever.
Probably the most important level 15 skills come from
Culinarian
Weaver
Leatherworker
Alchemist
The other crafter skills are good but those are absolutely necessary.
A good way to level is to make a macro that uses all of your durability on an item and just let it do most of the work. Here's a crafting macro generator if you want to try it out. It allows you to do other stuff in the background but still get some quality on the item to boost your xp per item
http://www.garytay.net/ffxiv_macro/
When you get to 30, you get a desynthesis skill to break down items into parts. If you use it while grinding out items it cane save you a ton of money in the long term. Keep in mind though that the skill has a maximum amount of skill points so you can only max 3 desynthesis trades and if you try do more it will just be kind of a pain in the ass later. Desynthesis at higher levels can be insanely profitable. I've been making about 1 million gil a day since the expansion breaking down vendor items for leather. It is also extremely expensive to level though.
