Its dark side? Bossy, inflexible, overwhelming, rigid, stubborn, their way or the highway, bad listeners, self-centered, vain, compliment seekers, envious (of other's success and social position mainly), loud, volatile temper, aggressive, goal-oriented (without considering others), no empathy, seductive (to increase their self-esteem), manipulative through drama, drama queens for no apparent reason, attention seeking ("I'm here, look at me"), tactless, invasive, self-righteous, aloof and cold when they feel their pride is hurt.
Bright side: loving, protective, make you feel important to them, optimistic (and it's contagious), driven, bright, decisive, express their love through physical contact (hugs, kisses, etc.), sexy, funny, self-ironic, encouraging, generous and selfless if evolved, noble, humble, ready to help.
But well, I'm a Scorpio stellium. I find Leos quite irritating unless they have Cancer or Virgo placements. However, the planets in my stellium are in the 5th house. My mother is a Leo Sun, Venus, Uranus, Pluto.
The above goes mainly for Leo Suns. My experience with other planets in Leo are the following:
Leo Moon: generous and caring, but needs attention and much pampering. Ready to help.
Leo Venus: affectionate but high maintenance, tends to be clingy and superficial. Generous.
Leo Mars: intolerable for me. Short-fused, egocentric, sarcastic and hurtful to make you feel inferior, insensitive, dramatic, inflated self-esteem. If they like you, especially in a sexual way, they will chase you and make you feel the sexiest person in the world. Tend to have very high libido, sex is explosive and intense, they exude sexual vibes. Quite kinky. The only thing I like about this Mars.
Don't have much experience with Leo Mercury.