Official kX Project Site -- News You can read all about them there.
Here's a full compability list:
kX Project -- FAQ (Frequently asked questions)
And check here for some screenies:
kX Project -- Screenshots
They are 3rd party built from scratch and share nothing common with Creative's own. This isn't something like PaX modified drivers, it's just a few coders that have coded for years in their sparetime as Creative open sourced specs/drivers or whatever for the Audigy card series (hasn't happened for X-Fi cards yet so obviously there's no X-Fi support). They are so configurable, you can add whatever plugins you want as long as there's enough resources on the card, they all use hardware acceleration (takes up resources from the card itself so no extra CPU juice is wasted) and the performance is very good, in fact I might have even gained 1 - 2 fps here and there for pc games.
The best thing is the awesome sounding bass output which you can extend an almost unlimited amount with either Timbre or 10-band EQ that are default plugins included in the pack and it won't start distorting despite raised it by even 20dB or more! lol The other nice thing is the pseudo surround sound that works amazingly well even for a stereo setup by the Surrounder+ plugin when enabling "surround" setting. The surround sound or 3D positional sound is so accurate I can shoot zombies hiding behind trees or behind a wall based on which direction the noise comes from in L4D or I can listen to footsteps or weapon noise in other FPS games to know where the enemy is. The best is it works very nicely with almost any games, no need for EAX support, it works even for very old games that wasn't intended to have any surround sound! Another scary thing when listening to some live rock concerts for example, it's like you could tell where the singer, the bassist, the drummer etc stands in relative to the source recording! xD
For me an Audigy card combined with kX Audio drivers is a better option than X-Fi or auzentech card at least, can't comment on ASUS Sonar series as I haven't tried them yet but I'm so satisfied with kX Audio drivers I bought a backup Audigy 2 ZS and Audigy 1 card whenever this Audigy 1 from 2001 fails.
64bit and Windows 7 support was quite recently added (maybe some 6 - 7 months ago or whatever) so there might be a bit bugs under that scenario but that's mostly with 5.1 setups if configured wrong (default settings won't do) but it works nicely with headphones/stereo sound as I've been using Win 7 64bit and haven't had any probs at all, in fact the sound quality is slightly better in Win 7 compared to XP. Even crisper/clearer highs and bass sounds also a bit better. I recommend Wave 4/5 output for headphones.