Q: Is Logic Studio 9 compatible with Lion ?

To put it simply; it works.

There are some 3rd party plug-ins that don't seem to work at all, but most can and will work in 32bit mode. If you're only using the plug-in found in Logic from Apple, they will all work in 32 and 64bit mode.

As Muzicman said, it does work.  However, I've personally encountered random crashes and quirks under Lion,  as well as some of my plug-ins failing validation.  It has to be said that I was expecting much worse.  I'm still using Snow Leopard for Logic Pro, and for anything else I'll boot into Lion.

Logic 9 1 4 is compatible with Lion

http://support.apple.com/kb/HT4807

It's usable but I wouldn't rely on it just yet.  You can be productive but certain third party plugins may not work.  For example Altiverb 6 is not compatible and failed validation, also Kontakt seemed to crash running in 64-bit mode (but it was okay in 32-bit).

I think if you mostly stick to Logic's own plugins and tools, you'll be safe.