I've been playing this game now for about 7 months, in that time I have never bothered using anything other than Mender Merchants to repair my gear, although the option to use Dark Matter's have been available to all who join FFXIV at Level 1, it's something I always thought there would be no need of.

Earlier I noticed how cheap
Grade 6 Dark Matters (for Equipment levels 1-60)
were, at 120gil a piece they work out cheaper to repair a full set of broken gear than merchants, but the huge benifit for all of you to use Dark Matter is how it affects the condition of an item, through Menders the Maximum Condition for your gear is 100% but by using Dark Matter it can go beyond that, currently my Level 60 White Mage Weapon has a condition of 146% after using my dark matter