The Senshi:
Sailor Mercury was the first senshi, besides Sailor Moon, to enter the scene. She is also a very unique sailor. Unlike the rest of the senshi, she does not 
have very strong attacks, but instead is an unproclaimed defender for the group. Her ice/fresh-water based attacks (see the mythology section for a short explination of her attack basis) are perfect for stopping or slowing down the enemy, but not for doing any considerable amount of damage.
Sailor Mercury is very helpful in that she provides a definite and usually reliable means of protection for the senshi. However, she is also helpful in that she provides the senshi with locations, statistics, enemy weakneses, and other bits of useful information while actually on the battle field. her VR Visor and mini-computer provide her with data that always proves useful to the other senshi. She is also a very intelligent girl and can, therefore, come up with good strategies that can be implemented quickly and effectively.
The Girl:
Mizuno Ami is, as has been stated, a very intelligent girl. She is the smartest in her class, and aspires towards becoming a doctor (a pediatrition I believe) like her mother. She transfered Juuban Junior High from a private school (refered to in the English dub as "the brain farm") so she probably is fairly wealthy.
Ami is rather introverted, and concentrates greatly on her studies. However, 
she is also very kind and friendly towards those who approach her, hence her almost instant friendship with Usagi. In the episode where Ami swims against Michiru, we see that it is in Ami's nature to hold back sometimes. This is, most likely, due to her desire to please others, and her willingness to make sacrifices (seen through her refusal to become an exchange student).
Relationships:
Besides the Rei-Usagi and Michiru-Haruka relationships, there really aren't many senshi-senshi bonds that are made clear. Ami in particular, doesn't seem to favour any one senshi's company over anothers. I do believe that Luna may be the character with whom Ami spends most of her time, but this isn't very surprising considering how much they have in common. I also believe, that Ami has a great deal of respect for and looks up to Michiru. The only other significant relationship that comes up is the one she has with a boy named Greg, who soon falls out of the picture and is hardly mentioned after the first season.
Opinion:
I do like Ami, but not very much. I am in strong disagreement with statements that depict her as cowardly. I believe it must take a tremendous amount of courage for her to go into battle knowing that her powers are not as strong as those of the other senshi. Ami's personality is very quiet and reserved, one that doesn't appeal to me very much, but I do like to see her interact with the other senshi on a deeper level.
Sailor Mercury's attacks constitute most of my favorites. They are very beautiful, even if water/ice isn't your favorite element.