My only 50 def is dark/rad. Was a ton of fun to level.
All the cool toys from Dark/ (as you note) and reasonable blasty fun with the /Rad.
City of Heroes was my first MMO, & my favorite computer game.
R.I.P.
Chyll - Bydand - Violynce - Enyrgos - Rylle - Nephryte - Solyd - Fettyr - Hyposhock - Styrling - Beryllos - Rosyc
Horryd - Myriam - Dysquiet - Ghyr
Vanysh - Eldrytch
Inflyct - Mysron - Orphyn - Dysmay - Reapyr - - Wyldeman - Hydeous
I've just started one. Best thing about it so far is Nemesis Staff and Sands of Mu hitting 95% of the time. 
Villains: Annie Alias, Dr. Amperical, Shade Golem, Knight Marksman
Heroes: The Clockwork Mime, Soccerpunch, The Fissioneer, Samurai Houston, Oversteer
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Dark/Rad was my first character, my first 50, and my most played character. I've also got a lvl 50 Rad/Dark corr.
They're awesome. That's all I really need to say.
I've got one at 50. Brutally effective and a blast to play. Need to get her out more.
Just started one today... working on the build, but here it is (no IO or anything too deeply thought out yet).
Hero Plan by Mids' Hero Designer 1.401
http://www.cohplanner.com/
[u]Click this DataLink to open the build![u]
Radical Dark: Level 50 Magic Defender
Primary Power Set: Dark Miasma
Secondary Power Set: Radiation Blast
Power Pool: Fitness
Power Pool: Leaping
Power Pool: Speed
Ancillary Pool: Dark Mastery
Hero Profile:
Level 1: Twilight Grasp -- (A)(5)(5)(7)(7)(17)
Level 1: Neutrino Bolt -- (A)(9)(40)(40)(43)(45)
Level 2: Tar Patch -- (A)(3)(3)
Level 4: Irradiate -- (A)(11)(19)(25)(34)(37)
Level 6: X-Ray Beam -- (A)(19)(25)(40)(46)(48)
Level 8: Howling Twilight -- (A)(9)(11)
Level 10: Hurdle -- (A)
Level 12: Fearsome Stare -- (A)(13)(13)(15)(15)(17)
Level 14: Combat Jumping -- (A)
Level 16: Health -- (A)
Level 18: Darkest Night -- (A)
Level 20: Stamina -- (A)(21)(21)
Level 22: Aim -- (A)(23)(23)
Level 24: Shadow Fall -- (A)
Level 26: Hasten -- (A)(27)(27)
Level 28: Cosmic Burst -- (A)(29)(29)(31)(31)(31)
Level 30: Super Jump -- (A)
Level 32: Dark Servant -- (A)(33)(33)(33)(34)(34)
Level 35: Neutron Bomb -- (A)(36)(36)(36)(37)(37)
Level 38: Atomic Blast -- (A)(39)(39)(39)(46)(46)
Level 41: Dark Consumption -- (A)(42)(42)(42)(43)(43)
Level 44: Dark Embrace -- (A)(45)(45)
Level 47: Soul Drain -- (A)(48)(48)(50)(50)(50)
Level 49: Oppressive Gloom -- (A)
------------
Level 1: Brawl -- (A)
Level 1: Sprint -- (A)
Level 2: Rest -- (A)
Level 1: Vigilance
As you've noticed, Dark Miasma has defense covered.
Radiation brings to the table lots of AoE attacks. Some of them are a little slow animating, true, but man oh man the procs you can slot in those babies. A defender with Achilles Heel in every blast? Yes, Mr. Defender, blast all you want, please!
I really like mine. Plays really differently from my TA/A (my other 50 def) and thus is very refreshing as I'm mostly a melee type of person.
@Viper Kinji
Currently working on:
Turtle Snapper - SD/MA/Ice Tanker
hah i remember this build being the uber tank-mage back in the day
if only i could find those demos theconfessor made
Yes, lol and i actually do awesome in pvp, love this build... use him for TFs a whole lot
How many of you have them and what do you think. Ive rolled one. I LOVE it really fun defender to play, great debuffs.Ran a LGTF last night fav moment was we had a team wipe on final av and large group was puled. I survived, i tped whole team to av and used howling twilight for a whole team rez, and mezed whole group. Was great.