RUN in this game is primarily a tank job. As a tank it excel at 4 main area: Status resist, magic damage taken, parry/blink tanking and gambit/rayke.
This job has much higher magic evasion and magic def bonus from job trait/gifts, so it takes less magical damage and will be more likely to resist status ailments such as para stone etc, even without using any JA. It also has very high parry rate, and very high fast cast, so it also excel at /NIN blink tanking the enemy since you can keep Utsusemi: Ichi up nearly full time.
It's currently one of the hottest endgame tanking job because it has the ability to boost BLM nuke dmg by using job ability Gambit/Rayke.
That being said, it has less HP than a PLD, and are considered to be harder to play than PLD by the community. If you enjoy a busy tank role job, it's a great choice. But if you enjoy hitting things as hard as possible, then I wouldn't recommend it.
According to my spreadsheet, DD RUN only comes close to real DDs such as WAR(no MS) when getting unrealistic attack buffs, and use aeonic + very top end gears. Even then it's still a tad below none MS WAR and if you don't have aeonic it's even further behind. On my spreadsheet a fully buffed at capped pDIF, Montante+1(best none REMA DD GS without using random augment system) RUN is at least 1200 DPS behind equally buffed/geared lionheart RUN. Not to mention the job has less offensive JA/spell/trait than most other DD.
In other words, if you're just starting out as a new/returning player, without awesome gears and unrealistic strong buffs, you aren't going to deal a lot of DPS, especially if you consider how much attack buffs a DD RUN needs which also aren't realistic in most pt.
If you enjoy hitting things hard and play complicated/busy job, I wouldn't recommend RUN, personally. The situation that RUN can perform well as a DPS role is just too niche. Most of the time you'll probably see an equally geared BLU winning parse.
However if you enjoy tanking and play a complicated/busy job, then RUN is a great choice. You don't really need awesome gear to tank well on RUN, as long as you know the job. The job as a tank performs well even with mostly 119 empy and cheap/easy to obtain gears.
If you're looking for a complicated/busy or hybrid DD job that DPS well in most situations, BLU or DNC is probably a better choice.