The problem I have with GK is a fluff to rule translation one. Grey Knights are supposed to be a chapter which specialises in banishing Daemons, however they just so happen to be able to banish everything else too
lol, that's how I feel too. Not that I'm complaining, though
I'm just concerned about the eventual Nerf-hammer.
A vanilla Grey Knight, if there is such a thing, rocks Str X in shooting at twice the efficiency of a regular marine in a distance game and still keeps the option to assault, then in hth has a matching Str X option with a weapon that cleaves through power armor.
Grey Knights gets termies at scoring units without pesky special characters/force org limitations. That's huge in 6th ed, where 2+ saves are the "new black", to borrow a phrase from high fashion.
The pre-measure game plus the psybolt ammo just works in tandem now. +25% stopping power with 2x effective mid range firepower with a serious buff in hth at a 33%-40% cost increase. I'll take that any day.
Tactics-wise, I'm finding the combo of GKSS with Warp Quake plus a Psyfleman very efficient. The Psyfleman gives the GKSS much needed long range support, plus very good anti-psyker support to boot. Warp Quake GKSS keeps deep strike drop pod assault meltaguns, fusion blasters, multimelta dreads, Sly Rambo, Trygons, Scourges, etc, away from the Psyfleman. The anti-deep strike bubble Warp Quake that can be created with a 10 man squad is pretty massive, 18" to 20"denial radius from the squad's center point is not impossible.
The Psyfleman, though not a Land Raider, is not made of tin-foil, either. He can take advantage of terrain plus has full firepower on the move.