The two-landraiders-GK-rush isnt an unusual list, but it's as risky as to a foot-slogging GK army. Fast moving and high mobility army such as Eldar and Tau will just dance around such an army all game and avoid combat with ease. While two Land Raiders do drastically reduce the amount of GK marines you can have on the table, but it also guarantee a second turn charge should the land raider survive its first turn by smart manuever and positioning. 500pts of Land Raiders equals to two extra 9-men GK marines squads with Justicar, but they will also have to spend 4 turns of walking to get into combat against most static army.. and god knows how many marines you're going to lose in that 4-turns of shooting.
If you ask me if I would take the risk of running 2 land raiders loaded with Gk marines inside, I would tell you a definite YES.. but it will be backed up by teleporting attacks of everything else in my army.. fast attack GK, teleport Termies, etc. I am under the impression that a pure GK army will lack the ability to fight an attrition war, anything that goes lightning fast in-and-out will be my style of playing my GK!