People really like using the soldier because he has the Rocket Launcher, the most familiar weapon to any quake veteran. Although the soldier is considerably slower than he is in Deathmatch mode, the rocket launcher still has a great range and blast radius.
Also the soldier has a great nail grenade ("+gren2"). It takes some time to arm, but clears out a room real efficiently. Although the nail grenade is fun to use, utilize your rockets more than the grenades. Predict your enemies moves and lead your shots accordingly. Aim real low, towards your opponents feet because direct shots are very rare. Blast damage is the best way to kill your opponent, especially against faster opponents (Spy, Scout). Be careful however, you have to reload after every four shots, so don't go buc wild and end up having to reload in the middle of a fight. If you run out, try using a carefully placed regular grenade ("+gren1") or drop a nail grenade ("+gren2") and find cover until reloading is complete. Also turn off autoim ("noaim 1"), so you can control your rockets.

AGAINST

When fighting a soldier count the number of rockets he uses, then attack when he has to reload. Beware of his grenades however! Don't ever fight a soldier in a closed area, long hallways always give his long-range rocket ability a distinct advantage. Use the soldiers slow speed against him, learn to circle-strafe against him, while throwing shells or nails at him. Also, jump when meeting a soldier, they usually rely on blast damage to kill you. But since you're higher away from the blast, the damage is reduced significantly.