GLF5 HAS NMAC WITH VFR CITATION DURING TERMINAL ARRIVAL.

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WE WERE IN VMC. DURING THIS PHASE OF OUR FLT WE RECEIVED A TA AND IMMEDIATELY LOOKED FOR OUR TFC ON OUR TCAS DISPLAY FOR REF TO POS AND ALT. THE TA TARGET POPPED UP BASICALLY FROM NOWHERE AND APPEARED IMMEDIATELY OFF OUR NOSE. BEFORE WE COULD ANALYZE VISUALLY OR WITH REF TO OUR INSTRUMENTATION; THE CITATION JET PASSED ABOVE US APPROX 50 FT. ABOUT THE SAME TIME THE CTLR SAID; 'ACFT X; YOU SHOULD HAVE TFC PASSING BELOW YOU.' WE ALSO; AT ABOUT THE SAME TIME; RECEIVED AN RA AND A DSND COMMAND FROM OUR TCAS COMPUTER. HOWEVER; ALL THIS WAS AFTER THE CITATION HAD PASSED. THIS ENTIRE EPISODE LASTED LESS THAN 5 SECONDS FROM THE INITIAL TA. ONCE WE GOT ON THE GND AND HAD TIME TO GATHER OUR THOUGHTS; WE CALLED APCH CTL TO INQUIRE ABOUT THE EVENTS. THE SUPVR ASSURED US THAT HE WOULD LOOK INTO THE ISSUE AND CALL US BACK. WITHIN ABOUT 1 HR WE WERE NOTIFIED BY APCH THAT A CITATION HAD DEPARTED UNDER VFR AND DID A HIGH PERFORMANCE CLB TO 5000 FT STAYING BELOW CLASS B AIRSPACE. THE ANGLE AND RATE OF CLB OF THE CITATION JET DIDN'T GIVE ATC A CHANCE TO GIVE US ACCURATE INFO AND IT DIDN'T ALLOW OUR TCAS SYS TO REACT PROPERLY EITHER. IT WAS A REAL FORMULA FOR DISASTER! IT IS OF MY OPINION THAT THE CITATION PLT USED POOR JUDGEMENT IN THE SAFE OP OF A JET ACFT IN A HIGH TFC AREA.