I've already gone into how much (and why) I love the mini-arcs that comprise season four of Enterprise and the Romulans Are Jerks arc is no different.
Right from the start, this one has a lot going for it. The arc follows a Romulan plot to spread dissent among pretty much all the folks who live within a warp-speed-stone's-throw from Earth but, in Starfleet fashion, Archer takes their plans in the exact opposite direction. One of my very favorite TOS episodes is Journey to Babel and Enterprise's salute to that one--Babel One--does a nice job of introducing the promise of peace and a history of unrest between the Andorians and Tellartites. Shran is awesome, his girlfriend is kick ass. The Tellarites are appropriately rude. Archer has finally come into his own as a diplomatic yet tough Starfleet captain.
The way this story, these cultures, these characters come together is well-constructed and satisfying. Shran's arc alone is captivating and the way he plays his part of this story is sincere and heartbreaking. The Romulans (who I've always inexplicably loved) are great and devious and totally just as bent on universe domination as they are later to come (though maybe not as good at it yet.) This arc fits beautifully into the rest of Enterprise and the rest of Trek and totally deserves a watch.