The meltagun is really the eavy hitter for the IG infantry. It's got the highest strength of any SW we can take, and one of the best AP value. Ideally, it's used for tank busting, but there are a host of other ways to use it.
Let's break down what units can take them, and how to use them.
CCS: This is always a good choice. The fact you can take 4 meltas, and have the shots hit on a 3+, means you'll hurt what you shoot. Packing a CC COmpany Commander in with them is also nice, since you can fire, then charge, unlike with Plasma guns. Keep in mind, though, you shouldn't have your CCS running arnound chasing tanks. Have them sit back with the gunline, and pop Termies or Deep Strikers that show up later in the game. The disadvantage of having a Melta CCS is that you're not taking things like Plasma Guns, which benefit from the BS 4 and the ability to take a Medic even more then Meltas.
Tip: Straken+3 Meltas+Medic. Toss in 2 bodyguards and Carapace, and you've got a party.
PCS: Another decent choice for Meltas. You're packing 4 of them, and if you take a Commander with a Pfist and meltabombs, that gives your platoon a lot of AT punch. The real downside is the the BS 3, which reduces the effectiveness of the unit. That's why I prefer to run Flamers in my PCS.
Tip: Al'Rahem and 4 Meltas. Have him flank and pick out the enemy commander, of a very expensive target, and kill it with preddijuce. You can also give him a Chimera, too, if you want a little more fun.
Infantry Squads: Don't take them here. The melta is a one shot per turn weapon, and the poor BS of the basic squad, combined with the fact you've only got one, means that it really isn't worth it. If you MUST deal with tanks, take meltabombs for the Squad Sgt.
Tip: None here, really. Best you can do is give the squad "Bring it down!", but that means you aren't giving the order to an HW Section.
SW Squads: Better then basic Line Squads because you can take 3 Meltas, but worse then a PCS because you can only take 3, and you can't charge after having the squad fire. The base cost is also higher then that of the PCS, and the PCS is mandatory, so you might as well be putting Meltas in it. I would tack on a SW team for meltas only if you have your PCS doing other things (like running flamers).
Tip: 2 Meltas and a Demo Charge will end pretty much anything, but combining them with Al'Rahem makes them even more lethal.
Veterans: This is where most people put their meltaguns, simply because it's the best option. Plasma is better in the CCS, because you back it up with a 5+ save AND feel no pain (instead of a simple 4+ save), flamers are kind of a waste of BS 4, and the GL is better for line squads. There are three basic ways Vets with Meltas are used. First, people will pack them into a Valk, and have them jump out and pop tanks. Second, people will often pack them into Chimeras, and have them do Drive-by atacks. Third, and this is one I use myself, they'll give the Vets Shotguns, and hold them in reserve. Once something nasty like Terminators deepstrike, I bring my Vets in, shoot, and charge. Giving the Sgt. a Pfist and a plasma pistol is even more lethal.
Tip: Carapace is good fo melta Vets, but Demolitions is even better, as you get a free demo charge out of the deal. Give Demo Vets some shotguns, pack them in a Valk, and have them go around shooting tanks and slapping meltabombs on things.
Stormies: Since Stormies are now almost exclusvly an anti-MEQ unit, so Plasma and Meltas are the order of the day. Plasma has the advantage of a lot of High Str shots, but Meltas have the advantage of being an "Assault" weapon, meaning you can fire the Hotshot Laspistols. This allows you to charge what you shoot, should you need to finish the job, or should you want to lock yourself in CC, so your expensive Stormies don't get their butts blasted off.
Tip: Behind enemy lines is the most useful Spec Op you can take if you're hunting for MEQs. If you're tank poping, grab a Valk and use Airborne Assault, or take Forward Recon if you're moving them on foot.
Rough Riders: Put meltas here only if you're hunting tanks. For Termie hunting, I like haivng the basic Pistols, so I can pick up the extra attacks for the Lances. Always couple them with Meltabombs for the Sgt, if you do go galloping after tanks, though.
Tip: Creed+Tactical Genius+ Rough Riders= Dead tanks and Devestator Squads. taking the Meltaguns also ensures you have some AP 3 punch after you charge the first time. Try charging an enemy squad to use the hunting lances, then go after armour.
That covers Meltas, who should take them, and how to use them! One thing I want to add though is picking your targets. If you're just wasting normal MEQs, take Plasma. It puts out more shots, so can chew through Tac Squads more easily. When dealing with Termies, I find it's useful to charge, to get the extra attacks in (and Termies usually have a Pfist, so you hit first)