Possibly repeating some of the points made above...
Frankly, I've never thought of the Centurion as of "the benchmark medium mech". They are quite specialized. For a small-scale combat I would prefer other mechs, but they do have some advantages. And I also like using those mechs as "bad guy's troopers" =).
CN9-A is good at accompanying non-jumping close combat mechs - like Hunchback, Thunderbolt, Orion, Flashman and the the similar ones. Compensate the lack of heavy armor and presence of the explosive ammo with range and friendly mechs serving as damage sinks. Provide fire support and initiative sinks for your main attack force. Avoid getting into the killing range until enemy is weakened, then join others, but still preferably keep at some range. Those AC/10 + LRM-10 can be quite handy. It's not quite good in the defense, unless you can avoid taking serious damage until the AC/10 can come into play.
CN9-AL has a bit more combat endurance, but it's still a medium mech, so it's role is generally the same. If all you have in this particular combat are mediums and lights, then CN9-AL is very good due to the ranged firepower and endurance. In a combat with the heavier mechs it still has to pull off somewhat to avoid taking too much damage.
CN9-AH is a jumping platform for the AC/20.
CN9-D is one of the best vehicle killers (mobile LB-10X). Good enough fire support mech. Avoid getting hurt until enemies are worn off, then crit them with LBX. Several other newer Centurions actually follow similar pattern, although I sill like the crit-seeking capabilities of the LBX.
CN-10B, despite it's standard engine and good armor, is still extremely vulnerable to the critical hits. I would prefer CN-9D. It's better to use the speed to avoid the incoming fire then staying there and getting killed.
Yen-Lo-Wang, RAC-turion are the like are skirmishers, sub-optimial, but good enough if used in the right moment.
Overall rate - 4 out of 5, good when used right, bad when the enemy doesn't present you such opportunity.