so I have my Future War Cult at Rank 5 I think, because I wanted the package you get from unlocking Rank 3,4,5 (although I ended up disappointed with those, because I wanted items but they kept giving me +1 Ascendant Material and +1 Strange Coin, except for one level where I got a FWC Fusion Rifle). I have New Monarchy at Rank 4 hoping for items, too, but rank 3 + 4 gave me Ascendant Material and coins.

My Vanguard Rank is like 12 or 13, because I got lots of nice equipment from them (had 3 out of 4 purple Warlock pieces just by playing my Titan that way, made leveling my Warlock alt a lot easier). But yeah, rep rank is a grind, as it usually takes 5-7 days or so to get one rank with very directed play at that goal, but you do get good shit from it, so at least that's worth it.