-Hellions:
take in as large a squad as you can with the special char who makes them scoring. this is their only real purpose. beasts are a better fast assault unit, warriors/trueborn do their shooting better.
-Wyches: better tarpit unit than assault unit against anything with marine saves. squad of 9 in raider, champ with agoniser, succubus with agonizer is the only way you'll pack enough out of wyches to punch through marine saves. if you need to deal with marines in hand to hand, you need incubi + archon combination.
-Scourges: expensive for their mediocre damage. mixed weapons stop these guys being too useful, shoot a tank and the carbines are wasted, shoot infantry and the AT weapons are wasted. keep these guys armed with splinter cannons and use for anti infantry. even then, trueborn in a venom do it better
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as for how people like to build armies in 6th:
generally tends to be lots of small units mounted in venoms. a couple of venoms put out an ungodly amount of AI fire for the points expenditure, though raiders are a must if you're rocking anything over 5 men.
-5 man wych squads with haywire grenades are popular to go stickybomb tanks/dreadnaughts etc, as are 5 man warrior squads with a blaster as troops. 10 man warrior unit in raider, with splinter cannon + splinter racks makes for a reasonable gunboat. dont see too many wracks as troops unfortunately.
-beasts units are popular because of their speed, lots of people chuck in a special character or two in here to milk some of the special rules that rub off on an already good unit.
-reaver jetbikes are pretty awesome. super quick, can carry AT guns and survivable (by DE standards). well worth a look at.
-elites vary from incubi to grotesques depending on personal taste for an assualt unit. many players dont take assault units now due to overwatch and strangely enough, premier DE units being too good. they often kill their opponent straight up and get left to take a full round of shooting! grotesques are my fave (cos i custommed the models) take an archon with phantasm grenade launcher, soul trap, husk blade and shadowfield for either squad and add a haemonculus if you take grots (furious charge is lovely on the archon). trueborn are a great option, blasters for anti tank or carbines and cannons for massed splinterfire.
-heavy support is a bit divisive. you'll often see x3 ravager gunships or x2 ravagers + flier of choice. not many people take the monsters because of their speed (or lack thereof) compared to the gunboats, but i really like them when you build your army around them (x2 talos, x1 chronos, next to x5 grotesques + supported with wracks presents a tough wall to shoot through). many armies struggle to deal with monstrous creatures, especially ones with FnP saves as well! Chronos is good here as an infantry support platform. his pie plate and flamer options are auto-takes as good use of both will net you 2 pain tokens a turn for nearby units.
if you do go for the monsters, then trueborn with blasters, reavers with heatlances/blasters can pick up the slack in the anti-tank department.
HQ is a bit divisive too, as mentioned, loads of people take special characters for multiple reasons (scoring hellions for example) but out of the mundane ones here's my pick:
Succubus, fairly meh. fine and cheap option for small games (just give her a venomblade and stick her in an assault unit of some kind) not too fussed but is capable of destroying whole guard/tau/eldar units single handedly. wont scratch marines much without an agoniser or power lance though.
Archon, gets expensive quickly but is the only option if you're taking grotesques or incubi due to his phantasm grenade launcher.
Haemonculous, not survivable so poor choice unless in really low pts games (he is a bit of a bargain). i like to keep them cheap with just the flamer option and bunker them in tough units like grotesques.
they rob assault units of their fleet, so don't put them in with wyches, however tempting it is to get them a pain token.
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units i never see: mandrakes. bloodbrides.
hope that helps!!