Yes, I've used it extensively.
It's very good indeed.
I also use Yahoo API, and Google API, .. but I prefer the MSN API.
With the API, you can return any result that you could get through using the normal MSN Search - and display it or use it in any way you like.
The easiest way to use it is through their Web Service in ASP.Net, but I have also used it in standard ASP. In ASP, or PHP, you have to create your own SOAP packet, which took me a while to figure out, but it does work. I posted the working SOAP packet in the forum here somewhere - do a search for it if you need it. Or I can post it again.
It's much easier in ASP.Net though. You just create your search packet, and then parse the XML results. The documentation isn't quite as helpful as it could be, but if you have any specific questions I'll do my best to help. I can't help with PHP though, only ASP or ASP.Net (VB.Net 2005).