The legion breacher siege squad does not have any true analogue in Warhammer 40,000, making it something of a unique Heresy era squad pattern.
One of the key differences for this unit is that their armour is hardened. As well as being void hardened, they can re-roll saves against blast and template weapons. This makes them ideal for the problematic siege situations that they will find themselves in. They're obviously going to be the primary troops selection for Zone Mortalis games for example. The boarding shield that comes as standard will help them stay alive in close combat (5++), but less so outside melee (6++).
The squad can take a larger range of upgrades than the tactical squad which also makes them quite interesting to build. But they are much more expensive to build than tactical squads. So the question for the fielding player is whether to have extra survivability in this option (or better flavour if you're Iron Hands, World Eaters, etc.), or more bodies with the tactical squad option. That said, here are a few sample builds.
Legion Breacher Siege Squad, 20 members, 4 graviton guns, legion vexilla, sergeant with power fist, artificer armour (425 points)
The reason for the sergeant's upgrades are the same as for the tactical squad. I like the graviton guns as an option as they provide for a nice toughness based mechanic for causing damage. And the sheer size of the squad is nice too!
Legion Breacher Siege Squad, 10 members, all with melta bombs, 2 melta guns, legion vexilla, sergeant with combi-melta, power fist, artificer armour (355 points)
Take with a land raider and go tank hunting. The extra invulnerable save will keep this squad alive longer than a similarly built tactical squad, but it is more expensive (of course!).
Legion Breacher Siege Squad, 10 members, 2 las cutters, legion vexilla, sergeant with power fist, artificer armour, one breacher charge, melta bombs (300 points)
An escort squad for the breacher charge and for good in your face damage to bulkheads! Probably take a land raider if you're not using this style squad inside of Zone Mortalis.