Sternguard are a unit capable of extreme versatility and when used properly, very powerful firepower. They are an Elites unit, so we'll be comparing them to other Elites as far as their capabilities...
WS 4, BS 4, S 4, T 4, W 1, I 4, A 2, Ld 9, Sv 3+
I think the part that surprises me the most is the 2 base attacks. They're clearly a shooty unit, yet they're as effective in close combat as a standard Assault Squad (1 base attack + cc weapon). This means that while you have them playing the midfield tearing up the opponent with your Special Issue Ammunition, don't worry about potential assaults. They're veterans, and can handle themselves.
To better assist possible assaults, you can trade your Sergeant's pistol for a power weapon or power fist ... allowing him to continue firing a Bolter when you're not assaulting.
Sternguard Veterans cost 25 points a model. While expensive when compared to normal Space Marines, they're still very cost effective compared to another Elites mainstay: Terminators (40 points/model). Sternguard Veterans is the Elites option for when you need decent firepower and can't afford Terminators. I would also argue that their Special Issue Ammunition gives them a versatility that even the Terminators don't have.
Special Issue Ammunition
Perhaps the cheesiest ability offered in the new SM codex, which is awesome for you SM players... but something you non-marines really need to be aware of.
The Sternguard come with several types of ammunition. EACH TIME the unit fires, it CAN CHOOSE WHICH TYPE it's using!
First up, they have available normal Bolter rounds: 24" S4, AP5, Rapid Fire
Dragonfire Bolts: 24" S4, AP5, Rapid Fire, Ignores Cover
IGNORES Cover! Who needs flamers, yeah? Rapid Fire and ignores cover!
Hellfire Rounds: 24" SX, AP5, Rapid Fire
SX means they wound automatically on a 2+.
Kraken Bolts: 30" S4, AP4, Rapid Fire
The answer to Eldar and Tau. 30" range to match Pulse Rifles, and AP4 now denies most of your base units their armour save.
Vengeance Rounds: 18" S4, AP3, Rapid Fire, Gets Hot!
With all the options available to them, there is virtually no unit type that they do NOT have an ammunition for! MEQs? Vengeance Rounds. 4+ armour save? Kraken Bolts. High Toughness(Tyranids/Wraithlord)? Hellfire Rounds. Enemy in cover? Dragonfire Bolts.
Yeesh. Given all the above come free with the unit, I almost laughed when I saw that you can have squad members replace their bolters for other special weapons. You have nearly everything here! Why give up that versatility for a flamer? I might understand a meltagun or the heavy options (Heavy Bolter, Lascannon) but by giving some squad members heavy weapons, you deny the unit the ability to fully maximize on those ammo types by not allowing them to move about the field as much. A squad of Sternguard needs to be constantly on the move, applying their specificly deadly firepower where it's needed most each round.
That said, the option to take combi-weapons adds some interesting capabilites while not simultaneously removing their option to use Special Issue Ammunition.
In a squad of 10 Sternguard, you can add in 4 combi-meltas for just a mere 20 points. This gives them the advantage of being able to deal with units they otherwise couldn't (vehicles, and 2+ armor saves.) (Thank you, Pcrackenhead)
Let's compare them to some other Elites.
Point for point, a unit of Sternguard are not only able to put out more firepower, but put out a specific type of firepower that denies an opponent's strength (whether that's cover, 4+ save, 3+ save, Toughness 8, etc.). Yet this does not in any way make the Terminators obsolete or a bad choice. What makes the Terminators worthwhile is the blend of firepower, close-combat ability and the 2+ save. They have a different role to play.
Dreadnoughts also occupy a different role than Sternguard: mainly as anti-tank/vehicle. If you're taking an anti-personnel variant, the Sternguard can probably do that job better, but again: neither option completely dominates the other in terms of viability. An Elites slot would be well spent either way.
Legion of The Damned
This is perhaps the Elites unit whose role overlaps the most with Sternguard. Their cost is comparable; they have the ability to Deep Strike, yet cost about the same as a Sternguard unit if you bought them a Drop Pod. Where they differ is that they trade the Sternguard's deadly ammo options for a 3+invulnerable save and Slow and Purposeful. Slow and Purposeful allows them to shoot as if stationary despite moving. Equipping them with some heavy weaponry would make a lot out of this, giving them strong firepower ... yet still not as efficient as the Sternguard's versatile options.
Neither option is "better" than the other in my opinion. It's a question of what do you want out of the unit. Strong and versatile firepower? Strong firepower and survivability?
Using Your Sternguard
First up, I would suggest maximizing the squad. If you're going to take a unit (goes for any of them) in your army, don't half-ass it. Maximize the squad size and/or take multiple copies of that unit. Make that selection viable and strong. 5 Sternguard might be a nuisance, but 10? Imagine the possibilities...
20 shots into a group of Pathfinders, denying them their cover save...
20 shots into a squad of CSM, denying them their armour save...
10 shots at 30" range into a Firewarrior squad, wounding on 3+ AND denying them armour save...
20 shots into a Wraithlord, all wounding on a 2+...
It's a scary thought. The key here is not only making it a viable option with numbers, but make sure that if you field them, that YOU KNOW THEIR CAPABILITIES. Memorize those ammo options! The last thing you want is a missed opportunity to seriously bring the hurt.
If your opponent fields one of these, I would suggest you take steps to either get rid of it fast, or get out of its LOS.
One last thing...
As if all this wasn't enough... if you take the special character Pedro Kantor as one of your HQ choices, he has a rule "Hold the Line!" which makes any Sternguard Veteran Squads you have count as SCORING UNITS.