Now I don't cotton to these new-fangled (i.e. 5th edition) dark reaper models. Really, with the bulletheads and big bat-wing ears it's even harder to take these guys seriously than it is the Night Lords. I like the simple, 3rd ed models; skull helmets and power armor. Nice, clean, gets the point across. But whatever.
1. Mini-Squad
I like the new rules for Reapers: a 3+ armor save, Slow and Purposeful (yep these guys don't get fleet or battle focus, so no shooting then running into cover!), reaper range finders; this is the most mobile version of Reapers to date.However. Three guys won't buy you much on the table for 90 points. A great gun for killing MEQs or even bikes at range, especially since with their default gear they will ignore jink rolls (reaper rangefinder). The ability to upgrade their guns to starshot (krak) missiles is even better. Look out skimmers!
There are cons to these guys, though. Six shots at S5/AP3 really isn't all that much once you start crunching the numbers. Also Slow and Purposeful denies them the ability to fire their heavy weapons (even as snap shots) and then dive into cover. The final nail in the coffin is their inability to fire overwatch. Yikes.
My gut feel is that 3 reapers is an easy kill point for your opponent, so unless you can hid them from opportunistic shooting I wouldn't bother.
2. Maxi-Squad
Hold up! For a whopping 300 points, you can now field a squad of ten of these guys! First time ever; in the past the squad was always capped at 5 models, including the exarch. At 10 models you are churning out 20 reaper shots or 10 starshot missiles(for an additional 80 points). This gives reapers an unparalleled ability to take down skimmer squadrons. Me like!Stick these guys in an Aegis defense line near a quad-gun or icarus lascannon and protect them; this is a relatively cheap way to counter aircraft. Yes they will die in droves to heldrakes, but so will everything else in your army. Here you at least have a fighting chance. Me very like!
The only difficulty with this setup besides keeping the enemy out of your face (remember, no overwatch!), is making back the points cost.
Side note: A lot of people think in terms of a unit making its points-worth back. I think that's a mistake; the value of a unit is not my unit cost = your your unit cost, particularly in the case of things like heldrakes. The value of the reapers in this case is the value of the unit they kill PLUS the value that enemy unit might have killed if left unopposed. End side note.
3. Exarch
The dark reaper exarch is a lovely little 10 point upgrade. On his own he's otherwise identical to a standard dark reaper plus the usual profile bump. He does, however, come with a whopping array of options.
Exarch Upgrades:
The usual 2-out-of-3 option limit. Night Vision is cheap, a 5 point upgrade that negates Night Fighting for the entire group, and easily a great value choice for picking off those choice targets at range at the beginning or end of a Night Fighting mission. Or Imotekh.
Fast Shot ups your exarchs shooting output but really depends on what equipment you choose. More on that later.
Marksman's Eye is another good one, granting Precision Shot on a 5+. Marksman's Eye pairs well with both other powers; Fast Shot granting more opportunities for the 5+ while with Night Vision your are hopefully getting more precision shots on enemy units before they can even see you.
Exarch Equipment:
There are a grand total of SIX different loadouts you can configure your reaper exarch with. Either default (starswarm) with or without starshot for 10 points, or a shuriken cannon for free, or and eldar missile launcher for 10. IF you opt for the EML you can upgrade it to flakk for another ten points. And finally the Tempest Launcher.
The Tempest Launcher looks good, but I just don't know. It has a shorter range than the rest of the squad, and while it has 2 shots at S4/AP3, I'm just not sure if it's worth the cost. Additionally, it has the Barrage rule so you can fire it out of line of sight, but of course if you are doing that, you lose the shots from the rest of the squad since the exarch does not come with Split Fire. Boo.
In the end it will come down to personal preference. I prefer the reaper launcher, with or without starshot based on what the rest of the unit is carrying. Shuriken cannons are a good option now with bladestorm, too. EML and Tempest are kind of meh to me now. Scattering blasts don't appeal that much unless I know I'm dealing with massed, light armored opponents.
4. Phoenix Lord
Oh Maugan Ra how I have missed you! I won't go through the long list of his abilities due to time constraints, but suffice it to say he's what exarchs SHOULD be; he gets Split Fire.My favorite use for him in a squad of as many reapers as you can afford: put them all in an Aegis Defense Line near the AA battery. Maugan Ra will handle the Interceptor fire: +1 shot, re-rolling 1s due to high BS, ignore jink, and skyfire adds up to one dead flyer as soon as it hits the table. Then on his own turn he can fire the Maugetar. Awesome. Expensive! But awesome.
My configuration of choice is Maugan Ra leading a maxed out squad of Reapers with no exarch. This will cost you a whopping 495 points, plus the Aegis and whichever AA gun you choose, but it is a solid, reliable unit killer. You just have to devote some reserves (reserve forces not "Reserves") to protecting them so they can keep killing.