I have all the d20 RPG material, and I consider it great. A good read, too, even if you don't intend to use the setting itself, ever.
It's not your average d20 campaign. Politics are important, political intrigue rampant, and the pen is mightier than the katana (So that courtier can't wield a weapon so save his life? Doesn't matter, since he could sic a hundred bushi on you.) Money isn't as important as in other games (most characters will probably be part of the samurai caste and be cared for by their lord)
I also got the 3rd Edition L5R roleplaying game rulebook that contains their own system.
I didn't have the chancer to try it out (I might do so one day, though), but it sounds very intriguing.
You use d10s all the way. It's not a bonus system as in D&D (roll d20, add bonuses to outcome, beat DC), but not a classical pool, either (like in WoD: look up your ability, add/subtract mods for difficulty, roll as many d10s: Every die with 8 or better counts as a success. 1 success makes the roll a success, 5 an exceptional success, and some rolls make further distinctions):
You roll a number of d10s, but only keep a certain number of them: Say you make a damage roll, the weapon has 3k2, your strength is 2, so you have 5k2. You roll 5 dice and keep 2. 10s are rerolled and added to the old result (so 10, 8, 4, 4 ,2 would mean you reroll the 10, get, say, a 5, so your two dice show 15 and 8, for a total of 23). You have to beat the TN (target number). If you want to do a great job (not just getting it done), you can raise. Each raise is a 5 point increase of the TN. You have to beat the higher TN (so in our other example, if the TN had been 20, raised to 25, the 23 would be a failure, even though you would have succeeded without the raise)
It's also not strictly level/class based like D&D, but not as free-form as WoD. You can increase most abilities, and get insight for the increases. If your insight points exceed certain limits, your insight rank (a bit like levels) increases. That allows you to train for the next rank in your school (more or less a class, though it doesn't limit yourself as much as d20 classes, it merely gives you one ability per level)