Before going any further, you need to go to the SWTOR site and read this. It's a post by a SWTOR developer detailing the item modification system that exists in Star Wars: The Old Republic. For those not in the know (which included me, btw) some pieces of equipment in the game are moddable, they have empty slots into which an item modification can be placed to give it stats. It's sort of like sockets in other games, but far more central to the functioning of the item. Sockets in other games were more of a bonus than anything, the mod slots in SWTOR gear are vital to them being useful.
Previously there were two types of moddable gear - gear that had nothing but mod slots, and gear that had some set stats and one mod slot. The most recent patch has apparently removed most of the latter and created more of the former with the intent of making using mods more focused. Since everyone gets some items that are fully custom through class quests we'll all be engaging in the system at least a little bit. As a force user, your first lightsaber is a fully moddable piece of gear.
I think the system is interesting, and as long as mods aren't too difficult to obtain I think it will offer a viable and fun alternative to the typical gear progression system. The fact that mods can be safely removed from gear is key, as it means really good mods can be saved and used on items you like the look of better, or saved for other items when you find an upgrade that would be better in it's current slot. With similar systems in other games I have frequently agonized over using a really good gem/enchant/whatever for fear of getting an item it would be better on shortly thereafter with no way to undo my choice. A fear that is usually born out too (as was the case with the +40 fire damage gem I slotted on my War in the North character's weapon, argh!).
The fully customizable items I've seen so far had quite a few slots, 3-4 if I recall. Getting to build your own item with that many choices sounds like fun, and I'm sure it's a system I'll use as much as I can. The fact that crafters can now create armor/weapons that are fully moddable will help. One thing that isn't clear to me though is that it's been stated you could use a fully moddable item from level 1 to the level cap if you wanted simply by updating the mods slotted in it. I'm assuming then there must be a mod that increases the dps of a weapon or the defense on armor? Otherwise I don't see low level custom gear being very useful no matter how good the stat mods on it are. I don't care that the lightsaber I started with has huge bonuses to stats if its base damage is still that of a level 9 item . . .
Mods can be obtained as loot, as quest rewards, from breaking down equipment, or through crafting. It sounds like a lot of possibilities, and hopefully it will be enough. I suspect the crafting skill that creates mods will be a profitable one overall, as it doesn't sound as useful as crafting your own armor or weapons, but in the end absolutely everyone is going to be using mods at some point.
I also really liked that if you see an armor drop that looks cool, someone somewhere has the recipe to craft that armor in a fully moddable version. If the system works well enough it will reduce the need for an appearance tab as you can track down the look you want and make the stats work by using mods. I hate looking like a frankenstein's fashion monster in these games, but not so much that I won't wear that piece of ugly armor with awesome stats. Being able to have the best of both worlds is something to look forward to, I just hope the execution is as good as the idea.