When should I use Grasp?
Matchups where you need to abuse it to win lane, can hit it easily and snowballing after is freelo. The only matchups I take it in are Rengar, Akali and maybe Riven.
Believe it or not, Thunderlords is good. Versus Zed. Don't take Thunderlords in any matchup other than Zed, it's just not as good as the other masteries against other champions.
Hard, poke matchups where you can't kill early. Think Quinn, Anivia, any ADC.
Every other matchup. It's just so good. Please note than when using Fervor your all in potential is insane early as well as scaling well into the late game. You can usually all in and get a kill at level 2 using Fervor.
Verus an AD laner OR an AD jungler, I use:
9x Armor Seals
9x Attack Damage Marks
9x Magic Resist Glyphs
3x Attack Speed Quintessences
Versus an AP jungler AND laner I use 9x Scaling Health Seals
Flash OR Ghost (Haven't playtested but Ghost seems great after the buff)
Teleport (Versus an AP top laner)
Exhaust (Versus an AD laner (Mid or top))
Ignite (Versus an AP mid laner)