I love Seraphim, I loved them in Dawn of War and even the models around now are actually my pick for best unit in the codex! Ok, maybe I'm a tad bias, but that won't affect my review in the slightest. First off, stat line, same as your average Sister of Battle and as far as wargear goes, exactly the same, minus the Bolter for a second Bolt Pistol. These girls were the original 'Gunslingers' and while other units such as Space Marine Command Squads and Vanguard Veterans have adopted the Dual Pistol Build as of 6th Edition and the introduction of the Gunslingers USR, the Seraphim are still queens of this form of combat. Seraphim can exchange their twin Bolt Pistols for either twin Hand Flamers or twin Inferno Pistol, two Sisters may take this particular upgrade no matter the squad size, while Inferno Pistols are melta and getting 4 Strength 8 AP 1 shots is so tempting as it is effect vs MCs, Heavy Infantry and Vehicles, the Hand Flamers are the way to go, the reason has something to do with their Act of Faith.
Special Rules wise, Seraphim are just awesome, They have the typical Acts of Faith and Shield of Faith special rules, however they also have Hit and Run, Seraphim Pistols (pretty much an early version of Gunslingers), Angelic Visage and their Faith power, The Emperor's Deliverance. Angelic Visage is the big one, it allows re-rolls of failed Faith tests and Shield of Faith saves, which means they almost always pass their Faith tests and have 6+ Inuvln. save that re-rolls isn't something to sniff at. Then comes the Act of Faith and reason why I take Hand Flamers, The Emperor's Deliverance allows re-rolls to wound when shooting... BY THE EMPEROR!!! this power is amazing and is one of the few that are actually aggressive.
All up, Seraphim are an awesome FA choice that can really hurt enemies if played right, their only downfall is the price tag that comes with playing them, remember they still are just Battle Sisters so if your not keen on spending a lot on a unit that can be cut down quick, even with Power Armour, then Seraphim are not for you, but they are by far one of the best units in the codex.