With GW releasing 40K rules for the new Custodes models (albeit without a way to field them quite yet, aside from Unbound; I am going off the assumption that that will be corrected soon), I think they have solid potential for competitive play. Looking at their stats/rules, they have the following:
-WS5 BS5 S5 T5 W2 I4 A3 Ld 10 Sv2+
-Guardian Spear (Shoots just like a Bolter and, in close combat, has the following profile: S: +1 AP2, Two-Handed, Block — Gives a chance to block one enemy attack per turn, if you roll higher than their To Hit roll) and Power Knife
Custodes can trade the Guardian Spear for Sentinel Blade (Power Sword with 12″ S4 AP5 Assault 2 shooting profile built in, which also can make Snap Shots at BS2) for free, which means you go down to S5 AP3 and lose AP2 (and the Block special rule), but gain +1A (since the sword is not Two-handed and combines with the Power Knife for two close combat weapons) and the ability to shoot and assault the same turn (unlike the Guardian Spear, which cannot assault after shooting, since it is a Rapid Fire weapon). More importantly, if you do NOT have a Guardian Spear, each Custodes can take a Storm Shield, giving them a 3++ and counter-acting a critical vulnerability, namely the lack of any Invul Save in their “natural” state.
Special Rules wise, the whole squad is Bulky, can Deep Strike, has Adamantium Will, is Fearless, and (most importantly) has Eternal Warrior! GK Paladins are dying with envy for these kind of stats! 😉 Seriously, though, this makes them quite flexible and reliable, with the option to deploy by Deep Strike or ride in Land Raiders, no danger of every failing a Morale check, and insurance that they will never lose both their wounds to a single attack (except it is a Destroyer attack, of course). One small interesting note is that all Custodes are Characters, not just their “sergeant,” meaning you can choose any of them to take a challenge (great for saving attached ICs) and also any of them can try and Look Out Sir! on a 4+ to preserve their wounds.
Finally, one Custodes in your army can trade his Guardian Spear out for a Custodes Vexilla Banner, giving Fearless to any friendly Imperium units within 12″ and +1A to any friendly Imperium. Combine all this with the Shield Captain “squad sergeant” (who is mandatory and has +1WS, I, and A above normal Custodes) and the option to add up to 5 more Custodes, at 10 MBs per model, and you have an expensive but potent unit, especially in close combat.
OK, now that we have gone over the basics, how do we get the most out of these expensive models? I think their primary weaknesses are lack of an Invul save (unless they drop Guardian Spears for Sentinel Blades and Stormshields), their lower mobility (Deep Striking aside), and their mediocre shooting (basically just Bolter shots at BS5). On the other hand, they strike at S6 AP2 with I4 and WS5, with 4A per model on the charge (5A if the unit has the Custodes Vexilla banner), they are T5 with 2W and Eternal Warrior to make them VERY survivable against anything that is not AP2/1, and they can mix up Guardian Spears/Sentinel Blade + Storm Shield combos to give them both hitting power and durability in the same unit.
Here are some ways I think to “force-multiply” them right out the gate and take them to the next level in terms of competitive game play:
 Azrael — Attaching the Dark Angel Chapter Master gives them all a 4++ (even with Guardian Spear), making them MUCH more durable without sacrificing their S6 AP2 attacks at full initiative. Additionally, if they are riding in a transport with him (say a LR Redeemer or a Spartan Assault Tank), then his 4++ is conferred to the transport, making it more 50% more survivable and thus more likely to deliver them to combat successfully. Finally, if Azrael your WL, then he can choose his trait, ranging from adding +3″ to the unit’s Run and Charge moves to giving the squad Furious Charge (so all the Guardian Spear Custodes are hitting at S7 on the charge, making them a real threat to most vehicles) to FNP for the unit if they are close to an Objective.
 Brother-Captain Stern — With guaranteed access to both Hammerhand and Sanctuary, he can make the already dangerous Custodes into almost invincible Close Combat monsters. Hammerhand makes their Guardian Spears strike at S8 AP2 (at I4 still) and even models with Sentinel Blades attack at S7 AP3. Better yet, Sanctuary ups any Custodes with Storm Shields to a 2++, allowing them to “shrug off” an average of 83% of all enemy AP2/1 attacks for the rest of the squad. Finally, he adds his own close combat power, especially his Force Weapon, into the mix and also gives the whole unit Preferred Enemy (Daemons) and enhanced Deny the Witch (which synergizes well with the Adamantium Will of the Custodes).
 Sevrin Loth — With guaranteed access to Endurance, Enfeeble, and Life Leech (in the Biomancy Discipline), he can double the survivability of the Custodes against anything that is not S10 (or has Instant Death inherently) via FNP(4+), he can reduce enemy Toughness by -1 (so S6 Guardian Spears “double out” T3 Space Marine, for example), and he can restore lost wounds to Custodes via Life Leech. Alternately, he can go Telepathy instead, with guaranteed access to Invisibility, again making the whole unit vastly more difficult to hurt via shooting or close combat. Finally, he is very solid Close Combat fighter himself, especially with his ability to have a 2++ save using the Armor of Selket.
 Ulrik the Slayer — While not as dramatic as some of the HQs already mentioned, he gives the entire squad of Custodes he is attached to Preferred Enemy, without any requirement to pass any Psychic Tests, and he also gives them FNP(6+), for a small buff in durability. Ulrik is not amazing as a fighter, but does have a BS5 Plasma Pistol and 5 x S6 AP4 attacks at WS6 and I5 on the charge, so he can add a little more “uummpphh” to the Custodes, especially against vehicles and units with 4+ armor or worse. Finally, if he is the WL, he also gives the unit Monster Hunter, which is quite solid for dealing with enemy MCs and GMCs that are more and more common in the game these days.
 Sanguinary Priest with The Angel’s Wing relic — A cheap way to give the Custodes FNP, without any need to roll for Psychic Powers or pass Psychic Tests. Additionally, the Angels Wing allows the entire unit to Deep Strike more safely/accurately and also makes it much less vulnerable to enemy Interceptor fire (mainly thinking of Tau here).
 Deathwatch TDA Librarian with Dominus Aegis relic — The DW Librarian can move normally with the Custodes or Deep Strike in with them, depending on the circumstances, and can also use any Psychic Powers he rolls to enhance them (imagine if he got Warpmetal Armor and cast on the Custodes squad, making the entire unit majority T6!). The Dominus Aegis gives the whole unit a 4++ if it does not move, which also activates on the second round of any combat (since they cannot move, by default, if they are stuck in combat), making the entire squad much more durable at a bargain points cost.
 Ordos Xenos Inquisitor with Rad Grenades, Ulumeathi Plasma Syphon, and Liber Heresius relic — The Rad Grenades reduce enemy Toughness by -1 on the first round of any combat, making the Custodes more potent in combat than they already are, the Plasma Syphon reduces the threat of Rapid Fire Plasma Weapons significantly, and the Liber Heresius relic allows the squad to Scout at the beginning of the game and gain Counter-attack and Hatred for one turn each later in the game, which can be quite useful in the right circumstances.
 Two Adepta Sororitas Ministorum Priests, one with the Litanies of Faith relic — The Priests gives the Custodes Hatred for re-rolls to Hit in the first round of combat (the Custodes already have the Fearless portion of Zealot), which is a solid buff in and of itself. More importantly, the Litanies of Faith allows both Priests to auto-pass their War Hymns tests, meaning that as long as they are alive, the whole unit AUTOMATICALLY gains Shred for its close combat attacks and AUTOMATICALLY re-rolls any saving throws (both Armor and Invul) in close combat… taken together, especially with a few Custodes with Storm Shields up front for “tanking” purposes, and you should be able to tear through even the most deadly opponents like Wulfen, Ork War bosses in Mega Armor, Hive Tyrants, GMCs, Iron Hands Chapter Masters with the Gorgon’s Chain relic, etc. etc.