In general, I like. I do, however, wish that the episode had been a little more subtle in its attempts to make us like the character, or at least not recruited the Doctor for the job. I preferred the approach taken in "Rose," in which the Doctor is almost entirely dismissive of Rose even as we learn to appreciate her. With Martha, there was a palpable sense of the writers checking items off a list of good companion attributes--observant, inquisitive, smart, cool under pressure, willing to step up in a time of crisis--and making sure we noted each one by having the Doctor comment or at least react to them. Towards the end, it felt a little as though Martha's wonderfulness was being rammed down our throats, which is something that Rose managed to avoid for most of the first season.
I am, however, surprised at the reactions calling Martha a beacon of calm and rationality, specifically in contrast to Rose. I'm not sure I know which Rose most of these commenters are talking about. From her first introduction, Rose is singled out for being level-headed in a time of crisis, and all the way to the end of her run she deals with problems in a methodical, rational manner. She is also, by the way, the center of stability at the heart of her family group, although that group is smaller than Martha's. In fact, I'd say that most of the differences between Rose and Martha are differences of degree, not kind. Rose was rational; Martha is more so. Rose was the caretaker in her small family group; Martha is the peacemaker for an entire clan. The only immediately apparent and meaningful differences I can detect between the two women are that Martha is not, as of this point, in love with the Doctor (for whatever value of 'in love' you want to give to the relationship between the two characters at the end of the second season), and that she is educated and middle class. I'm rather hoping that it's the former, not the latter, that is at the root of the many effusive reactions to the new character--which usually end up disparaging Rose either implicitly or explicitly.
Much as I enjoy the character, I am a little concerned about the nature of her fledgling relationship with the Doctor. With Rose, there was a sense that both characters got something meaningful out of the relationship. The Doctor needed companionship, someone to help him overcome the trauma of the Time War (as others have noted, the corresponding trauma of losing Rose has given Ten a much needed sense of depth). Rose needed a sense of purpose, some direction in a life that had previously been stagnating. At the end of "Smith and Jones", I couldn't tell what, beyond the fact that no one in their right mind would say no to the offer, would compel Martha to join the Doctor--what does she need that he can give her?
After a second viewing, however, I'm starting to wonder whether, in this respect, Martha isn't Rose's exact opposite. If Rose's life lacked direction, Martha's has it to spare. She's overwhelmed with responsibilities - she has to pay the rent, pass her exams, smooth her family's ruffled feathers, become a successful doctor, marry a suitably educated and wealthy man and have smart, beautiful children. If Rose was expected to know her place and be satisfied with it, Martha is expected to strive. It's easy to imagine how the chance to step away from these responsibilities might be very appealing.
The problem with this character arc is that however much the Doctor and his companions would like to pretend otherwise, Doctor Who isn't actually a show about the time-and-space travel equivalent of taking a year off to bum on a beach in Phuket. Being around the Doctor is practically a master class in taking responsibility. I'm not sure how Martha and the Doctor's insistence that she can hop on for a quick jaunt on the TARDIS and then step back into her life, completely unchanged, fits into this ethos, although obviously it won't be long before their arrangement becomes open-ended. It just seems a bit strange to me to start with a character who had already accepted the responsibility of saving her own small corner of the world long before the Doctor came into her life, and for whom the opportunity to travel with the Doctor is a chance to put that responsibility aside.