All this data is potentially out of date, and should be taken with a truckload of salt
The Harbinger is a good armor tanker, and is able to deal immense amounts of damage. It's battlecruiser role makes it tend to be a deadly support vessel. Finding it in fleets is not as easy as finding a hurricane, however it is not uncommon in well balanced fleets.
File:Azual skoll icon.png Know Your Enemy by Azual Skoll, The Altruist
At first glance, the harbinger is very similar to the hurricane - it shares an identical 8/4/6 slot layout, albeit with 7 turrets and a single damage bonus rather than 6 with a double bonus. The interesting thing however is that while the cane is more commonly shield tanked, the harbinger usually fits an armour tank. I suppose this isn't surprising given its amarr heritage, but I think it's also down to a few key differences: the harbinger is slower (not massively, but enough to make a difference), less cap stable (thanks to its choice of weapon system), and while it has excellent optimal range it lacks the flexibility of falloff. All of these make kiting that bit more challenging.
The standard harbinger is fairly similar to the armour cane above - 1600mm plate, trimarks, plenty of gank and mid slot ewar. Pulse lasers are the weapon of choice, backed up by a single neut (the harby has no missile hardpoints, so it's usually either a neut or an empty slot) and 50m3 of drone bay and bandwidth - enough for either 2 flights of light drones, or one of medium. Where the harbinger really pulls ahead of the armour cane is on range - with scorch it gets an optimal range of around 20km - that's full damage at a range where the armour cane would already be into its second falloff (meaning less than half its maximum damage). And that damage output is good - roughly comparable to that of the cane. It's tank is fairly solid - slightly stronger than an armour cane due to higher base armour HP and a small racial bonus to explosive resist. In all, it's a tough ship.
While less common than the armour variant, I still run into shield harbys fairly often. The setup is generally similar to the shield cane, although usually with more focus on damage rather than range mods, due to the smaller bonus given to optimal by tracking enhancers than to falloff. Typically you're looking at optimal ranges with scorch of 20-30km with about another 5km of falloff - enough to hit effectively anywhere inside point range with no problem (again, drawing a comparison to the shield cane remember that this is pure optimal where the cane would be in deep falloff, giving the shield harby a significant dps advantage at longer ranges). Tank is again similar to the cane, although you can expect it to be slightly softer thanks to lower base shield HP and slightly more restrictive fitting than the cane. The main difference as I mentioned is speed - the shield harby isn't slow, but you're looking at around 1200m/s compared to the cane's 1400m/s. It doesn't sound like much, but it's about the effect of an additional nanofiber for free, and combined with their slightly shorter overall range (the cane can afford to drift in and out of point range during a fight, whereas the harby has to stick rigidly inside its optimal in order to deal good damage), it makes the harby that much easier to catch. As with the cane, shield harbingers occasionally fit for range with beams rather than pulses, however this is generally much less common outside of gangs.
Fighting the harby is much the same as fighting the relevant cane fit, aside from the additional drones and the lack of a second neut. The mid-slot modules may be different, with more incentive to mount a cap booster due to the cap-use of its weapons (although I'd say they still aren't too common outside of active tanks). Don't underestimate the harby because it doesn't have the reputation of the cane - it's every bit as dangerous. It does however have slightly worse tracking than the cane (especially if the harby is fitting heavy pulses), and providing you aren't webbed you may gain something by getting in as close as possible. Because of the single neut and reduced tracking, smaller ships will probably have an easier time against the harbinger than the cane, but it will still be a tough fight. Killing drones may or may not be worth it, depending on how much of a threat they are to you - remember that if the harby is using lights it will have a second flight, and if its using mediums they probably won't do all that much damage against a small ship. If the harby is carrying light drones, be prepared for the second flight to be ECM. Thermal and EM are the damage types of choice against a shield harbinger, while kinetic and explosive are preferable against an armour tank (the harby gets a small racial bonus to armour explosive resist, which while still slightly lower than kinetic at base stats, may or may not be buffed with a hardener). Due to their strong damage output and moderate tank, harbingers are typically a good primary target when engaging a battlecruiser gang, particular if you suspect they are shield tanked.
Below are additional ships images. Click the image below for a larger version: