Grim, brutal, calculating, and the real masters of taking down defensive sieges, as well as sieging up locations themselves, the Iron Warriors seem to be a popular choice of army in 30k. And I can see why people like them: their background combines together elements of sheer brutality when Perturabo was rediscovered and took command, with I-almost-feel-sorry-for-them because Olympia rebelled against their rule, and what-choice-did-Perturabo-have-but-to-join-Horus. This, plus the fact that they're one of the most spread out forces (due to defending certain locations) means that there are many, many loyalist Iron Warriors knocking around the Galaxy at the time that Perturabo sided with Horus. This makes things highly interesting to say the least, as much with the Alpha Legion, one can never quite tell if an Iron Warrior detachment is going to be loyal or traitor -- at least at the outset of the Heresy.
The rules for the legion reflect well their ability to take on overwhelming firepower and take down enemy positions of all kinds. The first rule they have is Wrack & Ruin. This is a great boon which means that they will never take tests for running away if they suffer lots of casualties as a result of incoming fire. Plus they get to re-roll pinning tests. These two facets are amazing in and of themselves - recall that in 30k, ATSKNF does not exist, hence the chances of units fleeing off the board are a serious concern. Not so for the Iron Warriors when they take incoming fire (but yes: they certainly can flee due to close combat). Immediately, this probably means that the Iron Warriors should be thinking about a focus on ranged superior fire power. On top of this, the same rule gives Wrecker to the legion's melta bombs and grenade attacks - perfect for this legion really!
The second (and final) special rule is called The Bitter End. In brief, it allows the Iron Warriors player to go to the full game length rather than stop early (should they desire). This can be a boon under certain conditions, but may never come in to play.
Overall then, the Iron Warriors are a very solid force with very little negatives. In many ways, they kind of archetypes for Legion forces - a baseline for others to compare with. Sure, the Iron Hands have arguably better boons, but they also have significant draw backs by comparison. The Iron Warriors are all positive and are capable of most things in the game, even if they're going to be mostly a shooting force.
Other rules include being able to field Warsmiths (see later), having access to Shrapnel Bolts (heacy bolters gain pinning, but also AP5), and Cortex Controllers (for controlling Castellax and the like). The latter is particularly interesting for the Iron Warriors, and fluffy as well.
Their unique rite of war, The Hammer of Olympia, allows them to make disordered charges even after rapid firing their bolters, all their vehicles gain more armour and they can even field more heavy support! This is amazing and should seriously be considered by any shooting mixed-arms Iron Warriors force.
Overall, I consider the Iron Warriors to be an excellent example of what a legion could be, and a force to be reckoned with. Played to their advantages (shooting, mixed arms, and not falling back under shooting), they are a force to be reckoned with. I can certainly see their popularity both from the rules and the background.