So, here's the skinny on this one and it's companion, the Dragonborn booklet. Are they going to make or break your character or campaign? No, not by a long ways, especially if you're a decent role-player or DM. However, the $10 price tag for these 32-page gems are really worth it, if just for the background information given in both booklets about the ancient empires of Bael Turath (tieflings) and Arkhosia (Dragonborn). The ruins of both of these fallen empires are the generic settings for many adventure sites in the modern 4th ed. world.
This product follows the outline laid out in PHR:Dragonborn in that it starts with fluff and history of the race, outlines some of the different noble houses of Bael Turath and their roles within the empire. It follows this with a power-focused (arcane, divine, melee, primal, psion) analysis of Tieflings and which classes within those powers the race excells at, complete with background story hooks and paragon paths. The booklet ends with feats for the three tiers, quests specific to tieflings, and an epic destiny: Heir to the Empire.
Overall, I would recommend this for any player interested in fleshing out their Tiefling character, or any DM that is even remotely interested in the Bael Turath/Arkhosia history and settings. 4 stars for not being an absolute "must-have", but still an excellent product that I wish they would do more of for the less-standard races.