In many ways similar to the 40k tactical squad analogue, these troops will no doubt be the core of many 30k armies. But there are significant differences. They do not have the modern "And They Shall Know No Fear" rule, nor do they have the combat squads ability that can be so useful in tournament play when you don't know what army you'll be facing.
Instead, they have the "Fury of the Legion" special rule. This is the rule that everyone has probably heard about when Betrayal first came out. In short, they shoot their bloters and bolt pistols twice in a shooting phase. But they cannot shoot again next turn due to the massive amount of bullets unleashed. Nor can they overwatch in the current turn either.
If we want to take full advantage of Fury of the Legion, then a fully expanded tactical squad makes a lot of sense. And indeed, you'll probably see many 30k net lists with a full 20 members each in their tactical squads to take direct advantage of this. But this has to be balanced against the want / need to take rhino transports, drop pods (depending on the rite of war in play) and so forth.
Here are two builds to consider. The first is a regular one to take advantage of Fury of the Legion where needed, the second is a Despoiler style squad.
Legion Tactical Squad, 20 members, sergeant with power fist and artificer armour, legion vexilla (285 points).
I think this is the baseline squad. We're almost forced to take a power fist in 30k over a power weapon since the opposing sergeants (or better) issuing challenges will no doubt have a 2+ save or some kind of invulnerable save. Perhaps a power lance at a push, but probably not. Take melta bombs if there are some points to spare. Add on an apothecary for bonus survivability / objective capturing power-up!
Legion Tactical Squad, 10 members, additional chainswords each, sergeant with power fist and artificer armour, legion vexilla, nuncio-vox, melta bombs (220 points)
The tooled-up 10 man tactical squad. Notice that the extra close combat weapon has to be paid for. Place in a rhino and have some fun.
Of course, the temptation as always is to really tool up each and every squad. But there, there is something to be said for a plain 20 man un-upgraded squad for 250 points