I would highly suggest downloading MOO1 and giving it a try. Yeah, the Mrrshans are very aggressive, and they also happen to be the only race with two mortal enemies (Alkaris--cats and birds, and Sakkras). Their bonus is also arguably the least useful; they are "excellent gunners" so I think they get like a +2 on targeting, which seems useful, but compare that to the Alkari advantage of +2 to shields. If Alkari run up against Mrrshans, the Alkari can essentially fend off the Mrrshan advantage, while their slightly inferior lasers take out the crappy Mrrshan shields. Defense always comes first, even if you don't have great weapons, you will win battles if your shields protect you from better guns. I think the Mrrshan's are also poor researchers with the possible exception of weapons, which is actually the least useful as it confers no additional benefits (i.e. computers aid in spying, construction in ship building, etc).
So yeah, they are just overall awful. My favorite (aside from Humans and Psilons) are the Meklars because they get +2 factories per population. At the outset this is a disadvantage because factories are expensive and it costs so much to clean up all the pollution from the extra factories that essentially you are paying a huge premium for extra factories that confer no production benefit because of the resources required to keep the planet habitable. However, when you get some good eco techs at the beginning of the mid-game they become an absolute terror. AI Meklars favor Huge ships and stacks of small ships so large they can make you cry. If you can get a good eco restoration, and factories down to 5 BC per instead of the 10 BC they cost at the beginning of the game, then you're on easy street if you have a good start.
With all that, however, the humans remain the easiest to win the game with. They are good researchers, which helps, but most importantly everyone loves them, making diplomatic victories almost easy if you keep good relations with the other races. It also helps to understand stuff like "planetary invasion of your border systems doesn't mean the AI is at war with you, but if you do it they will go to war with you" and "enemy of my enemy diplomacy".
2 years ago, I was waaaay into MOO; I'd go to message boards and read game reports from good players and ****.