Valkyrie
The Valkyrie and its derived craft are a series of Navy VTOL-capable gunships/dropships capable of carrying Infantry/Veteran/Stormtrooper squads into battle and provide covering fire. Because of their sizable engines they can travel at considerable speed, evading enemy fire that would otherwise cripple the ship.
Valkyrie
The standard Valkyrie is only lightly armed compared to the others. Sporting a single Multilaser or Lascannon, two Hellfury Missiles or Multiple Rocket Pods, and possibly two sponson-mounted Heavy Bolters. Its main feature is a transport capacity of 12 men, who the Valkyrie can carry across the battlefield fast and in relative safety.
Vendetta
The Vendetta fills two roles. Aside from its transport capabilities of 6, it carries three twin-linked Lascannons to enable it to act as a dedicated vehicle and flyer hunter as well as a dropship, which generates a lot of rage as it can be taken in SQUADRONS. The Vendetta spam people feared never really occurred. It's hard to reliably shoot more than twice a game against routine lists and its reduced transport capacity limit it to Special Weapon Teams, Scions and Command Squads.
Vulture
Foregoing its passenger bay for a single massive engine, aside from it's nose-mounted Heavy Bolter it can carry any combination of Lascannons, Autocannons, Multilasers, regular missile launchers, Multiple Rocket Pods, Hunter-Killer missiles, Hellfury Missiles, Heavy Bombs, and even a single twin-linked Punisher Gatling Cannon as its primary weapon.
Some would call it the "Flying Blender", relating to how the big-ass air intake of the engine is right behind the cockpit. Seriously, one proper hit to the cockpit and the pilot is red mist. To invoke maximum nerd rage, use AH-64 Gunship model at 1:35 scale to represent this model if you dont have 150+$ laying around.
Sky Talon
A Valkyrie that had its weapons and crew compartment scrapped in favor of four magnetic clamps that can carry either two Drop Sentinels or a single Tauros or Venator.
In Dawn of War, this is the vehicle that drops off prefabricated Imperial Guard building components, as well as parts for the Baneblade at the Mars Pattern Command.
In Dawn of War 2, it drops off turrets, personnel, and vehicles.
While these flying heavy bolters lack real purpose as transport (noting that tauros's are fast vehicles, and drop sentinels can deep strike, consider taking them for their Multiple rocket pods option: At the same points cost as single taurox, and with missile pods they can really ruin light infantry's day.
Reasons to Rock a Few Valkyries/Vendettas
6th Edition flyer rules, and Grav Chute Insertion. These things have three hull points, 12 armour front and sides, and unless your enemy brings aircraft or skyfire, it takes 6's to hit her. They can move up to 36" and fire their entire arsenal (and ho boy, what an arsenal it is). As if that wasn't enough, with grav chute insertion, their passengers can deep strike to any location the V flew over. No need for the V to land.
Speaking of landing, if things get a bit too hairy for manouvering a flyer, say a cities map with a lot of tall buildings, they can drop into hover mode and dance on the spot. A useful little trick.
The downside? Well they're pricey models. The Vendetta runs even pricier since you have to buy the twin-linked lascannons in resin from Forge World (MONEY FOR THE MONEY GOD, DEBT FOR THE DEBT THRONE!). They also take up a lot of space, so put some cash aside for model cases.