in 30K, Rule of Cool trumps whatever usefulness a model or unit may have.
As Ashiraya stated, they are WAY overpriced Assault Marines. They have some nifty rules- but I'd be weary of things like Burning Decent where all models can be affected, they always get Hammer of Wrath, and the grapple axe thing is pretty cool. But if I was worried about their effectiveness on the table top, I'd probably take a Destroyer Squad with Rad weapons instead.
If I was a Word Bearers player though, heck yeah, I'd take 'em. They are the iconic unit of the Word Bearer's and no one else gets to take them (except Alpha Legion... boo hiss).
As far as tactics- use 'em like Assault Marines, which is what they basically are. Be careful about where you Deep Strike them, or you could hurt your own Legion. Probably best used in a multi assault with Terminators that can't sweep, but the Ashen Circle can and reduces the fall back roll of the losing side thanks to the grapple axe thingy. They could be good in conjunction with a unit of Khorne Daemons. The red guys with the swords? I forget the name. Best practice with these guys would be as a second line, multi-assault unit.
If you were worried about posts value and the effectiveness of the unit, you would probably never take a lot of the things that are available in 30K. Which is why the Rule of Cool exists.