They are PRETTY. YES. THEY ARE. Side view. Coat hanger bracing FTW.
Another side view.
A cookie if you spotted the ingredients already. It's not an exhaustive list.
1) Old-school Hive Tyrant
2) Dragon Wings from WH Fantasy
3) Pair o' Venom Cannons from Warriors
4) pair o' scything talons from the trygon/mawloc kit.
The limbs were generally chosen for sake of scale. The general scale of the thing was built with transportability in mind. The wings are pinned on, as I simply could not make magnets work that well. They're secured to the base at two points; there's the coat hanger and there's an additional point on the tail, be it the base or the jutting building wreckage.
First off, the Harpy is fast, as it's basically monstrous jump infantry. It can get a good 12" move each turn and then fire off both of its weapon systems. However, synapse means that you will probably NOT use it to rush across the field, and frankly there's no need to close with the enemy that much. 18" is just fine for infantry-hunting, and 36" is fine for shooting at larger targets. Use the speed not to advance, but to set up shots.
Honestly, you brought this thing because it's got guns. It starts with a twin-linked stranglethorn cannon, but I highly recommended paying 10 points to go to the twin heavy venom cannon. Why?
Simple: suppressive fire against vehicles. With twin-linked, the gun's got a base 50/50 shot of landing where you put it, just from rolling a hit with the scatter dice. More refined odds involve 2d6-3/ (size of the vehicle) and are a bit too tedious to put here. S9 has a good chance of glancing, and even with a -1 to damage from a penetrating hit it's likely to at least keep a vehicle from shooting, and will do damage 50% of the time on a good pen. Mostly, though, it's to shut those frakking tanks up for a bit. Use the mobility to line up flank shots if necessary, and you can always try to ping multi-wound T4 guys in a pinch.
In closer, you should've equipped Cluster Spines. Large blast, S5, AP-. Quantity over quality of hits, but it wounds most infantry pretty readily.
In really really close, you get the once-a-game Harpy Crap-over. You just have to move over an enemy model, and then you can barrage d3 S4 AP4 Large Blasts on top of them. Hilarity can ensue.
A word of warning: bad rolls on danger-close shots CAN hit you. As you are T5 with a 4+, it's reasonable to assume you can wounder yourself, and I kinda helped a harpy kill itself with its own large blasts.
Well, as a monstrous creature you ARE ignoring armor, and rolling 5+2d6 against vehicles. You get two base S5 hits, so pick your fights carefully: go for depleted units or go in with friends. You won't do much more than polish off a couple non-melee guys by yourself. Still, you do have Sonic Screech, which translates to assault grenades and making the enemy go at 1/2 initiative (rounded down), so if you can get buddies in there it's a good time to go to town.
Even so, I probably wouldn't try to fist-fight a dreadnought unless it's wounded or minus the Dreadnought CCW, since you'll get maybe one good penetrating hit on it in melee.
This is where some folks start quibbling about the 170pt price tag: you have four wounds at T5 with a 4+ save. At danger close, you can harm yourself with your own spore mine offal. Autocannons can ding you up, and a rail gun will break you in half.
I've got four answers for that:
1) in a pinch, hide behind another MC or grab cover if you can find it (or, say, a venomthrope).
2) ...so there's ONE ARMY that can insta-gib you at range. One. Crap happens. Are you seriously going to make your decisions based on ONE graphic mis-match? That's being a wee bit disingenuous.
3) Make sure harpies aren't the only monstrous creatures in your army, because, um, well, they won't like the attention.
4) when you get down to it, stuff like missile launchers and lascannons will wound you on a 2+ and take away your armor save ANYWAY, so how's the T5 and 4+ change that? Other than autocannons and rail guns, you're more or less identical to other MCs in terms of heavy weapons (...excepting heavy bolters, I'll grant, but they're not too common other than dakka-preds).
Harpies are mobile suppressive fire that get nastier up close. They're snipers with a point-defense system and some assault power. Use them to suppress/harass armor at range and from the flanks (since it's one of TWO guns that fire more than 24" and can harm vehicles in the 'nid book). If infantry closes, unload spore mines and cluster spines on them, and watch the wounds pile on. Take them as part of a shooty army (IE: consider Tyrannofexes, and/or hive guard/Venomthropes) to augment your long-range vehicle-harassing capabilities.
They're not all bad, but they're not auto-includes in all 'nid armies.