Hitmonlee isn't terrible—his main problem is that he is outclassed by Machamp. Still, he gets the best Fighting move in the game, and makes quite an impact in UU with his nice Speed.
The kicking counterpart to Hitmonchan, Hitmonlee is the only Fighting-type (or Pokemon, for that matter) that learns Hi Jump Kick, which other Fighting-types in RBY can only hope to have. Just try sending him out against Normal- and Rock-types, or something with weak defenses, and Body Slam/Hi Jump Kick away. You really have to be careful sending this thing out, since he will get 2HKOed by most physical and special attacks. Sending him out against a paralyzed Pokemon is ideal, as with some paralysis support, he can somewhat work a little better. Hitmonlee should really just be used in UU, as he has no place in the Psychic-dominated OU metagame.
Meditate can be considered to raise Hitmonlee's Attack. He is capable of making some Normal Pokemon switch out, and is reasonably fast, so you might be able to pull it off despite his pathetic defenses.
Checks and Counters
Most Psychic Pokemon destroy Hitmonlee by resisting his main attack, Hi Jump Kick, and launching assaults on his pitiful Special. Exeggutor and Slowbro are good choices, but even more fragile ones like Starmie can work; watch out for paralysis on the faster Pokemon, though. Gengar is completely immune to all of Hitmonlee's attacks except Seismic Toss and works similarly to the Psychic-types. Moltres and Zapdos also resist Hi Jump Kick and can launch strong special attacks or, in the case of Zapdos, super effective Drill Pecks.
In UU, you might want to be careful. Mr. Mime could scare it off, but in the long run, it can easily get worn down due to its poor defensive stats. Tangela and Golduck are good bets as they are some of the most defensive Pokemon in UU.