In the current metagame, Sisters of Battle hold a rather low point in the competitive ladder of armies, it being generally accepted that they have only one competitive list. This, along with their old codex and expensive metal models, makes them an underplayed and rare army, but somehow their popularity persists anyway. Everyone knows Sisters of Battle, and whether they make fun of them for being outdated or respect them for their unique playstyle, everyone has an opinion on them. That's all well and good, but just being popular doesn't mean that Sisters get played. So, why should you play Sisters of Battle?
#1: They're Unique
While there are any number of Marine Equivalent armies out there (Space Wolves, Blood Angels, Dark Angels, Templars, Smurfs...), and Sisters at first look like an MEQ army, there's a distinct difference between the deadly Sororitas and the mighty Astartes. The first, and most obvious at a casual glance, is their statline. According to the stats, Sisters of Battle are simply Imperial Guard Veterans in Power Armor, with Bolters instead of lasguns. They fit somewhere in the middle of Guard and Marine, but stand out distinctly from both thanks to Faith.
Most people will agree that the shining point of a Sisters army are Acts of Faith. Whether they're making them stronger, faster, more durable or more deadly, Battle Sisters have a wide variety of options open to every squad (with the purchase of a veteran superior!) which allows a Sisters force to adapt and change as the needs of the battlefield demand, something not many other armies can boast. And who doesn't love 3++ saves and AP1 Bolter shots? Acts of Faith add an entire new dimension to the game.
#2: They're Aggressive
Sisters are, without a doubt, a shooting army. Their Strength, Toughness, Initiative and number of Attacks does not lead to assault at all, and yet, unlike other shooting mostly armies, the Sisters of Battle demand an aggressive style of play that would cause Tau to falter and collapse. This is because Sisters of Battle, almost regardless of squad, have a sweet spot in which they are extremely deadly.
Within 12" Sisters of Battle are one of the most dangerous armies in the entire game. Double Tapping Bolters, Meltaguns and Flamers of all sizes fill their ranks, and with their Acts of Faith all of these weapons can be treated as AP1 on To Wound rolls of 6. Marines cower, even Terminators shiver when a Rhino full of Battle Sisters pulls up in front of them at point blank range. Sisters of Battle reward aggressive movement and careful positioning, and their best anti-infantry weapons ignore the cover saves so prevalent in 5th edition, removing the safety of ruins or forests for most armies.
#3: They Look Cool
When you think of Marines, what do you see? Boxes with guns and tank treads and lots of aluminum cans with bolters, all identical, moving across the battlefield. Some armies manage to individualize troops a little (I'm looking at you Space Wolves) but generally speaking, appearancewise, A Marine is a Marine is a Marine. Sisters of Battle, however, have a very striking and unique appearance. Clothed in holy vestments with windblown hair, or wielding impossibly large eviscerators, Sisters of Battle stand out from every other army in their high gothic style.
Even their vehicles look different, despite being based on the same Rhino chassis as the Space Marines. Immolators have their huge bubble turrets and the mighty Excorcist is instantly recognizable thanks to the wrought iron pipe organ protruding from its back. Just look at a Penitent Engine or a Seraphim and compare it to a Dreanought or an Assault Marine and the difference becomes clear.
#4: Their Theme
Holy warriors beating face on the enemy with the power of their faith. That's the Sisters of Battle in a nutshell. They are an army that cleanses and purifies, bringing the holy light of the Emperor to all heretics and nonbelievers (that means everyone). Whether you think that idea's amazing or just wanna smash their faces in, everybody has a reaction to a holy crusade of Bolter Bitches rumbling through town lighting everything on fire in the name of God. It's just human nature.
So, in conclusion, I'd like to say that I personally love Sisters of Battle (Obviously) and while they may not be considered competitive right now, they're definitely fun to play. there's no standing around, there's no prolonged combats that go nowhere. It's brutal, it's in your face, and most importantly it's ON FIRE. Say it with me now, kids. Heavy Flamer. Love it. Cherish it. Bring it home to meet your parents. Use it to burn them as Hereticus Extremus.