PLT OF A BE58 ON LNDG IN IMC HAS DIFFICULTY CTLING THE ACFT DUE LONG LNDG AND WET RWY. RWY EXCURSION RESULTED.

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I WAS FLYING THE LOC RWY 25 APCH INTO THE UNI ARPT IN ATHENS; OH; IN A BEECH BARON 58 IN IMC. THE FLT WAS CONDUCTED UNDER PART 91. I WAS THE PIC FLYING IN THE L SEAT WITH A COPLT WORKING THE RADIOS AND 2 PAX ON BOARD. THE AWOS RPTED THE CEILING AS 600 FT OVCST WITH 2 1/2 MI VISIBILITY. THE CONDITIONS ON THE RWY WERE WET DUE TO LIGHT RAIN AT THE TIME. THE MINIMUM VISIBILITY REQUIRED ON THE APCH CHART WAS 1 MI AND THE MDA OF 1240 FT WITH THE FAN MARKER GETS THE ACFT TO 475 FT AGL. UPON REACHING THE MDA WITH ABOUT 20 SECONDS UNTIL MISSED APCH TIME; MY COPLT RPTED RWY ENVIRONMENT IN SIGHT. WHEN I LOOKED UP FROM THE INSTS; I SAW THE REILS AND A COUPLE OF SECONDS LATER; I SAW THE RWY. I TOLD MY COPLT TO CANCEL IFR AND I EXTENDED FULL FLAPS AND REDUCED THE THROTTLES TO IDLE AND INITIATED A SLIGHT FORWARD SLIP TO MAKE SURE I GOT DOWN WITH ENOUGH RWY LEFT TO STOP. (I HAD ALREADY EXTENDED THE LNDG GEAR AND APCH FLAPS EARLIER ON THE APCH.) THE RWY IS 4201 FT LONG AND I TOUCHED DOWN ABOUT 1/2 WAY DOWN THE RWY; LEAVING ME WITH ABOUT 2100 FT TO DECELERATE AND STOP. THE RAIN MADE BRAKING DIFFICULT AND I ADDED FIRM BUT NOT FULL BRAKING PRESSURE SO AS NOT TO LOCK THE BRAKES; AND FULL BACK ELEVATOR FOR AERODYNAMIC BRAKING. THE ACFT DECELERATED VERY POORLY AND SLID OFF THE RWY ONTO THE GRASS. 2 RWY THRESHOLD LIGHTS WERE STRUCK AND A 1/4 INCH NICK ON 1 OF THE L PROP BLADES WAS THE ONLY DAMAGE TO THE ACFT. HUMAN PERFORMANCE CONSIDERATIONS THAT COME INTO PLAY INCLUDE MY DECISION TO LAND WHEN I BROKE OUT OF THE CLOUDS IN THE RAINY CONDITIONS. ALSO; ONCE I TOUCHED DOWN; I WAS FACED WITH A DECISION OF ADDING PWR TO TAKE OFF AGAIN AND RISK HITTING SOME TREES OFF THE DEP END OF THE RWY OR ATTEMPTING TO STOP. GIVEN MY HVY LOADING (2 PAX; ABOUT 90 GALLONS OF FUEL AND ABOUT 70 LBS OF BAGGAGE); MY SPLIT SECOND DECISION WAS TO STOP. IN RETROSPECT; HAD I DECIDED TO GAR; THIS INCIDENT MIGHT NOT HAVE OCCURRED. THANKFULLY; THERE WAS ONLY MINOR DAMAGE AND NO ONE WAS INJURED.