I myself have recently been playing on the PTR's considering the patch is (rumored) to be coming out next coming tuesday.
But anyways, I've cleared the 5 man instance, halls of reflection. I myself, as I can say; BLOWN AWAY!
Blizzard really has worked HARD on the frozen throne instances, and I'm not seized of amazement.
Exiting the instance, a event happens in which Prince of darkness himself, Arthas comes to raid on your parade.
Doing so, he calls upon his minions and Lady sylvannas windrunner aids you on escaping arthas considering if you even go NEAR arthas he'll tear you to pieces.
I thought to myself, god dang tomb raider all over again.
So I started running away from arthas as of everyone else did. BOOM big old ice wall. Everyone's panicking (i.e.: wtf zomfg we're dead.) Lady Sylvannas windrunner begins shooting the wall, myself thinking, " Well yeah. She's a big old city boss but she's shooting at a 2 story tall wall expecting to take it down. Whatever blizzard have your fun." Minions after minions start coming after you along with arthas (he's walking slower than ever) so basically, you've gotten to have to have ranged > melee. Due to the fact if Arthas is 5 yards of you, he eats you alive. (Not literally but basically.)
At the end of the instance arthas begins to choke and choke sylvannas with his darth vader powers. 1 minute within him choking her, luckily Thorims hammer (or your factions equivalent) ship comes in to save you, shooting the rocks above arthas supposedly crushing him, in which I doubt it. Afterwards, they give you a boost onto the ship, and you sail away, giving you your achievements and there's a chest that gives the loot also.
Fun instance. Something I'd never expect BLIZZARD to come up with.
IMO for the instance, I give it a 4/5.
Also, for you eager ones. Here's a trailer.
[yt]http://www.youtube.com/watch?v=WO1QJOooexQ&feature=player_embedded[/yt]