I'm planning to potentially TR my level 20 into a TWF build, and I'm debating between a fighter kensai or ranger tempest.
The main thing Tempest has going for it are its tier-5s: Dual Perfection (full stat bonus to offhand damage), A Thousand Cuts (combine with Haste Boost for burst DPS), and Dance of Death (+200% Strikethrough for 10 seconds). DoD is still the only way to add Strikethrough to dual-wielding AFAIK, which is what enables a Tempest to shred mobs faster than most TWF builds.
Regular ranger gets Evasion and the full Favored Enemy options; Dark Hunter gets trap skills and up to 5d6 sneak attacks free.
The main drawback to melee rangers is you need to invest heavily into two trees to have both a crit range & multiplier bonus; 40+ Tempest / 24 Deepwood Stalker is my usual spread. That doesn't leave a lot of points for other things. Also rangers don't get a Battle Trance, which means either investing in one of the Universal options or splashing cleric / FvS for Divine Might or Will, costing you the Tempest capstone. [Which isn't great but is still extra DPS and survivability.]