While the Caladius is being released by Forgeworld for 30K only right now, I think it is highly likely to be added as an option for the Agents of the Imperium codex (which will include Custodes) in 40K in the future. Based on that possibility, here are some initial thoughts on the tactical employment of this advanced combat vehicle, if it does indeed port over to the 40K universe:
In terms of stats, the Caladius is a Fast Skimmer with BS5 AV13/13/11 and 3HPs. It also comes with POTMS, Deep Strike, and Outflank (the 30K version also has Flare Shields and Grav-Backwash, but I doubt that will be included in a 40K version). In terms of weapons, it has:
 TLed Lastrum Bolt Cannon (36″ S6 AP3, Heavy 3, Heliothermic Detonation) on its hull
 TLed Iliastus Accelerator Cannon (60″ S7 AP2, Heavy 3, Rending, Rapid Tracking, Heliothermic Detonation) on its turret (Note: Rapid Tracking allows the Iliastus Accelerator Cannon to ignore Jink saves and Heliothermic Detonation allows both weapons to add +1 on the Vehicle Damage table for penetrating hits and force a Toughness test for Unsaved Wounds, with any model failing that test suffering Instant Death).
Finally, the Caladius can take Armoured Ceramite for 4 MBs and/or Extra Armor for 1 MB as upgrades (the entire vehicle without upgrades is just shy of 40 MBs in price).
In terms of tactical employment, this is basically a more expensive, more capable Sicarian Predator tank. With BS5, TLed on both of its weapons, and its status as a Fast Skimmer, the Caladius can move 12″, shoot both of its weapons (each at a different target, thanks to POTMS), and have 97% accuracy for all of its shots. AV13 on its front and sides mean it is quite durable, as long as you keep enemy forces away from its rear arc, and can ignore S6 and below shooting attacks from the front/sides (this is especially true if you take Armoured Ceramite for Melta immunity). Finally, if you need to get it into a specific firing position (or to grab an Objective), it has superior mobility options from the ability to Deep Strike, Outflank, or move 30″ in a single turn (12″ normal move + 18″ Flat Out for being a Fast Skimmer). All in all, you pay quite a lot for this vehicle, but in return you get a VERY capable vehicle all three key classifications of mobility, durability, and firepower.
Speaking of firepower, I think there are several very good targets for the Caladius:
 Bikes and Jetbikes — With the ability to ignore Jink saves for the Iliastus Accelerator Cannon, coupled with TLed BS5 and S7 AP2, you have a VERY good chance of killing two to three bikes a turn with this weapon, despite their T5 and access “built-in” cover saves. Add in three BS5 TLed more shots from the Lastrum Bolt Cannon and this tank can reliably kill (or at least reduce to just one or two survivors) an entire Biker or Jetbike squad every Shooting Phase (the long range of these two weapons, coupled with the ability to move 12″ and still shoot, makes this even more effective). If you are able to successful cast Phase Form on the Caladius, then it gets even better at this roll, ignoring both LOS and enemy cover saves with BOTH of its weapons.
 Monstrous Creatures and Hero ICs– Combining the high strength/low AP of both its weapons and the built in superior accuracy makes the Caladius quite good at chipping wounds off of MCs and ICs, especially at long range, before they can get in close to shoot and/or assault. What really makes this tank shine in this roll, however, is the Heliothermic Detonation rule; while T5+ MCs are unlikely to fail a Toughness test, if you inflict unsaved wounds enough times in different shooting phases, the odds are the will and, on that one time that they do, they lose any FNP they have (i.e. Riptides) and they also lose all their remaining wounds in a single shot. Against T4 and below ICs, failing a Toughness Test is even more likely (casting Enfeeble on the target can make this even more probable), which means that Warlords without Eternal Warrior have to be very wary of the Caladius.
 Medium/Light Skimmers — Ignoring Jink makes a big difference here, as does lots of accurate S7 and S6 shots that chip off HPs rapidly (or cause an Explodes! result on a roll of 5+, in the case of a Penetrating Hit from the Iliastus Accelerator Cannon). Many Skimmers also tend to use mobility and long-ranges to avoid enemy shooting, but the Caladius can often outmaneuver and outrange these opponents, thus negating these tactics. Finally, if using Deep Strike or Outflank effectively, the Caladius can get good flank or even rear armor shots, making even its S6 AP3 TLed Lastrum Bolt Cannon a threat. Basically, all Eldar, Dark Eldar, Tau, Necron, and Astartes Skimmers have to be very careful when an enemy Caladius in on the field.
 Heavy Infantry — Units like Tau Battlesuits, Centurions, Terminators, Meganobz, Wraithguard, Lychguard, Ogryns, Obliterators, and (ironically) enemy Custodes are all excellent targets for a Caladius’ weapons load-out. Combine this with the new Null Zone power, to reduce enemy Invul saves by -2, and you kill several dangerous/expensive enemy heavy infantry models each turn at quite long-ranges (i.e. for example, a Caladius could reliably kill and entire 3-man Crises squad each Shooting Phase, which would be pretty devastating for a Farsight Enclave army based around lots of these).
In terms of survivability, there are a couple of ways to enhance the Caladius’ chances of surviving enemy attacks directed at it. First, you can use a friendly Sky-Shield Landing Pad to provide it with a “free” 4++ save on top of its AV 13/13/11, without having to Jink (and so lose most of its firepower). Second, you can cast Warpmetal Armour on it, bringing its stats up to AV14/14/12 (so basically a Land Raider from the front and sides). Finally, you can repair it with the Reforge power (and grant it IWND in the process) and, if you have to, Jink for a 4++ cover save if you are desperate. Probably the best defense for this Grav-Tank is to use its range and mobility to keep it out of range/LOS from the most dangerous enemy weapons and take Armoured Ceramite to protect if from enemy meltacide squads.