I've played the roleplaying game, and the card game (quite extensively).
I think they're excellent. I love the setting, and the background, and the rpg is great fun and obviously much more involved in the story. There certainely was a d20 version but I think that's 2nd edition. The current version is a d10 system they call Roll & Keep and it works very well for the setting etc. The closest thing in d20 is probably Oriental Adventures
It's plus points; it's a great system for the setting. It's can be very deadly, which is about right for that sort of game, the dueling mechanism is excellent, the spells system is good, and the background and setting material, as you'll know is exceptional. I think it leans towards more roleplaying than combat compared to a d20 system. The art work is fantastic as well.
There are bad points as well. The core rulebook was poorly laid out, and I still find it difficult to find some things in there after playing for quite a while. There's also simply not that much support for it. Some adventures would be useful, for example, particularly for people new to the game, but it's all homebrew stuff normally based roughly on the L5R fiction etc.
I'd recommend it and I've had great fun playing it, and if you're a fan of the setting already then it's all the better.
The other place to check might be
Heroes of Rokugan a kind of Living Rokugan persistant world. I know there are games in various places in the USA and also in Southampton in the UK, and they might be a good introduction to the game.