Like 2bighorn said, ASIO is mainly used by the better sound editing packages. I think some of the better DVD movie players for Windows may also use it.
Creative's ASIO drivers are actually pretty good. Creative's hardware was not worth using for sound editing until the Audigy 2 line of cards were released.
What ASIO brought to the party was very low latencies amd the ability to by-pass the Microsoft sound mess, er mixer. Important when you are trying to sync up multiple tracks in an editing environment and when you need the exact sound you originally recorded.
In the sound card industry there have been the gaming sound cards and the editing sound cards. High end cards from Lynx or RME, for example, are not really good gaming cards, and are targeted at sound engineers for editing. How many gamers would even consider blowing between five and nine hundred dollars for a sound card? So those high end companies are not concerned with DirectX gaming support. Instead they focus on the ASIO drivers as well as the best sound production and hardware feature set best suited for editing. Most of them also have support for Linux, but not for Vista.
Microsoft sabotaged the sound API (in the name of DRM) in Vista which is making it the anti-choice for sound editing operating systems. In order to prevent Vista from munging the digital sound when using the ASIO drivers, you have to jump through some hoops. If you are not careful Vista will grab the sound and modify it. ASIO drivers no longer prevent the operating system from interfering with the sound stream, which is the one of the reasons ASIO came into existance.
