As a piston powered fighter aircraft?
Once the lateral instability issue was resolved over mid to late 1944, it was reportedly excellent - fast, long ranged and easy to fly across the envelope. Several test/trials pilots considered it more maneuverable and easier to fly than the Griffon powered Spitfires.
Eric Brown noted in 1946 that lateral instability was still not ideal, and that the ailerons were somewhat ineffective, hampering its rate of roll.
The A&AEE praised the aircraft for its cockpit layout (which they considered should be the future standard), as well as for its ease of maintenance, servicing and ability to turn around (refuel/rearm) quickly.
As a weapon of war? It was too little, too late.
Given the need for the rear fuselage/tail section redo, the MB-5 wouldn't have been ready for service until mid 1945 (at the earliest). It didn't really offer a significant advance over piston powered aircraft that were already in service (Spitfire XIV and 21, Tempest V/II), starting production (Hornet and Spiteful) or on order (Fury). It was also already clear that the jet was the way of the future, so starting production of yet another piston powered aircraft design - even a superlative one - didn't make any sense.