Building on what Slips has said above, I played a gunline list against Word Bearers last night, and my Tactical Squads were invaluable. I would have lost hard if I'd tried to run Stone Gauntlet and used Breachers instead.
My list -
Polux w/ Command Squad (3 bods, 2 x power weapons, 1 x Fist, 3 x Combat Shields)
2 x 10 Man Tactical - Nuncio, Melta bombs (1 x Sarge w/ Artificer & Power Fist, Rhino on one squad)
1 x 10 Man Breacher - Nuncio
2 x Predators (Lascannon Sponsons)
1 x Achilles Alpha
1 x Contemptor (LC, DCCW)
2 x Apothecaries
10 x Gal Vorbak
2 x 10 Tactical Squad (1 Apothecary each, with Kor and the Chaplain attached to either squad. Rolled Hatred for both squad with Dark Channelling)
Admittedly at 1500 points I probably didn't bring as much in the way of Troops as perhaps I should have, but I wanted to use what I've got painted minis for, which meant an armoured list, and that ended up working pretty well for me. Another thing that worked pretty well for me was the fact that he rolled Life Eater on the Mysterious Terrain table and lost three of his Gal Vorbak before the game started!
Basically, he ran/fleeted towards me and I shot at him, which was hardly unpredictable.
Some notes from my experience (these will not be earth-shattering, as this was my first game in about four years. They're as much for me as any one else!)-
MVP - My most effective unit was either the Polux squad or the Achilles-Alpha. The Achilles-Alpha essentially accounted for Kor Phaeron, an Apothecary and a Tactical squad by itself in two turns of shooting, netting me something like six VP in the scenario we were playing, as well as other incidental kills (3 Gal Vorbak and a wound on his Diabolist), but Polux was the game-changer. I had him and his Command Squad arrive on Turn 4 from Deep Strike, and in that turn he Combi-Melta'd the Contemptor, survived a charge and challenge from a very angry Gal Vorbak Dark Martyr, killing him in the next round of combat while the squad took care of the remaining Gal Vorbak and the Chaplain, and then they moved on to his remaining tactical squad, killing six on the charge with Polux challenging and instant-deathing the Diabolist in the last roll of the final turn with his special attack, which meant I'd been able to table the Word Bearers, which I felt pretty good about!
Alternate MVP was my Rhino that delayed his Gal Vorbak from reaching my lines by a turn by parking itself in a conveniently annoying spot and thus giving me another round of shooting at them.
Make room in your list for Apothecaries! Seriously. The difference in casualties between his squads and mine simply for having that extra FNP save was ridiculous. Admittedly he was rerolling a load of dice on charges due to Hatred, but if I'd had them too, the Apothecaries would certainly have made my Tac squads more effective. As it was, I had a single surviving Tactical Marine at the end of the game, with the Breacher and other Tac squad completely wiped out. Not good enough!
Shatter Shells are wonderful - Comparing the effectiveness of the Quad Mortar's shooting on the Achilles to the Thudd Gun, which doesn't have Shatter Shells, the Achilles wins hands down. You don't have the option of taking one in a LR squad, as far as I know, but in smaller games where you're less likely to get many dedicated anti-tank units, the extra points you pay for an Achilles-Alpha over a regular Achilles seem very much worth it. The Thudd Gun by itself simply isn't going to do enough damage to warrant inclusion in a list, and if I'd dropped it I would have had points for an Apothecary with some upgrades.
Bolters of Glory - I totally, embarrassingly forgot that you can't shoot next turn if you use Fury. This was a badness. However, +1 BS to Overwatch when you're being charged by Gal Vorbak is a thrice-damned lifesaver.
Changes for next time - The Predators probably didn't do enough to justify their inclusion without some changes. If I'd found the points to include my Executioner Cannon it would likely have been way more use against the Gal Vorbak than the standard loadout, which would have let me focus the majority of my shooting onto targets. I'll likely drop the Rhino and the Thudd Gun next time for either two Apothecaries or one and the Executioner. That said, I may also drop a Predator and add more bodies to my Troop squads.
Moral of the Story - If you're insistin' on a Fistin', Polux is your man.