The Imperial Fists, the praetorians of Terra and the rulers of Inwit. The rock and the foundation upon which the Imperium has been constructed. Loyalists to a fault. All these things and more are probably true of this unique legion.
Background Material Evaluation
Much of the background for the Imperial Fists is exactly what we have come to expect from this legion. One that is capable of defending fortified positions with ease, and one that is also capable with the not-so-humble bolter and bolt pistol.
Of course, there is also the whole rivalry going on with the Iron Warriors. In many ways, the Imperial Fists are the mirror of the Iron Warriors: skilled with building and ripping down fortifications and masters of shooting. But unlike the Iron Warriors, the Imperial Fists have a strong emphasis on being as solid as a rock. Therefore, what I have previously written about the Iron Warriors being a baseline force to beat applies to the Imperial Fists as well. These two legions are the ones that other legion players should be testing their legions out against. If they can defeat either (preferably both), then I think the other legion players are going to do well overall in many 30k games.
Legion Rules Review
Just like the Iron Warriors, the special rules for the Imperial Fists are all generally very positive with few drawbacks. The first one is Disciplined Fire. This is probably the one that 40k players might be expecting - something akin to bolter drill. For this rule, all marines get +1BS for using bolters and bolt pistols or their heavier variants. Very interestingly, the Imperial Fists heavy support squads gain tank hunters in addition to this as well. This is a great boon to the legion and one that they're going to be known for.
Note quite the counter-balanced I think it should be, the Blood and Honour rule means that this legion must issue challenges. I don't think this should have beed added to, to be honest, but it has been: these characters must re-roll failed hits. This gives such an edge to the likes of Sigismund when he doesn't really need any more encouragement.
Unshakable Defence means that the legion is stubborn when in fortifications or cover. This is a huge boon as stubborn is at a premium cost in 30k.
Finally, The Bitter End special rule that can force a game to go to six turns if an opponent wants it to. Not much of a negative here arguably.
Overall, these rules suggest that the Imperial Fists legion player should be looking to the bolter, to shooting and limited close assault in specialist units, combined with fortifications and cover to win their games.
Rite of War
The Stone Gauntlet rite of war is one that is modelled around miniatures that have shields; specifically boarding shields, or storm shield. Note here that the Vigil Pattern Storm Shield IS a Storm Shield (hence the name). In such units, the men gain +1T for being in coherency with at least two similar men. This is huge. A toughness bonus makes these marines almost as good as Plague Marines in some ways. Add in an apothecary, and this rite of war creates some serious monsters on the table top that will be very hard for some legions to remove by force. In addition, the Shield Charge rule grants a hammer of wrath rule to shield-armed squads. Naturally, this is balanced up by having to take breacher squads as the compulsory troops selection.
This rite of war argues strongly for many breacher and terminator squads with Vigil shields (see below), but also backed up with some longer range firepower and (or) tanks.
There are several items of unique war gear available to the legion. Like the Salamanders, the Imperial Fists have been developing shield technologies of their own. The Vigil Pattern Storm Shield grants a 3+ invulnerable save to any one who has it, at a penalty of costing extra points to equip it. For a base level standard Tartaros terminator squad, it'll be 75 more points thanks. For Cataphractii, 50 extra points. Hence they're not cheap. But they could well be worth it!
Complementing this is the Teleportation Transponder. For a per squad cost, terminators (and terminator characters) can gain the deep strike rule! Recall that terminators do not automatically gain deep strike and you'll see how good this potentially might be.
The Solarite Power Gauntlet is a potential replacement for thunder hammers, coming in like a regular power fist but master crafted. They're good in some ways (additional attacks when paired with bolt pistols for cheap), but I think I like the thunder hammer more for the concussive special rule to be fair.
Finally, the Imperial Fists (and, as it happens: the Blood Angels) terminators gain access to the Iliastus Pattern Assault Cannon. At the time of the Heresy, the assault cannon was a prototype weapon. Although it is heavy 4 with rending and S6, there is a small chance of malfunction - which means that it cannot be used ever again in the game. This is probably not much of a drawback, as by the time it'll malfunction, the terminators will be in close combat.
The Imperial Fists seem like an incredible army to have. Their strengths lie in their bolters, their terminator squad upgrades, and (with the rite of war) breacher squads.
Depending on which build one wishes to do, I think the Imperial Fists player can take on most in the game. For me personally, I would be erring on the side of building a couple of terminator squads with the special equipment (yes - that mean the Vigil Storm Shields) to make them unique, as well as taking full sized tactical squads to have an amazing BS5 fury of the legion attack. I would also be taking heavy support squads for the tank hunters rule. If I'm not taking the rite of war, I'd be tempted by taking a fortification as well (depending on the opponent and mission - if known in advance). Add in some tanks (vindicators, rhinos, land raiders, Sicarans) and its a solid army to run and beat.
Variants would include elitist terminator builds perhaps with pride of the legion or orbital assault, breacher builds with the Stone Gauntlet rite of war, bolter heavy builds, and almost anything in-between.