The first thing to note about first impressions of Juliet and Romeo in William Shakespeare's Romeo and Juliet is that they will vary considerably. A professor who has been teaching Shakespeare for twenty or thirty years will have a very different perspective on these characters than most students first encountering the play.
That being said, I find them both rather silly, self-centered, and impetuous teenagers. Romeo's infatuation with Juliet at first sight does not strike me as romantic, but rather as a sign that he is not yet capable of forming lasting relationships. As the play opens, he is pining for Rosaline. A few hours later, after seeing, but not even talking with Juliet, he says:
Did my heart love till now? forswear it, sight!
For I ne'er saw true beauty till this night.
If he is that fickle, and that susceptible to simply seeing a pretty face, the likelihood that he would have been a faithful husband strikes me as fairly low.
As for Juliet, she seems somewhat more thoughtful than Romeo, but still impatient and lacking good judgment and consideration for her family.
I am currently teaching Romeo and Juliet in my English I class right now, and my students and I just recently had a discussion about your question. I was surprised to find out that many of my students felt the same way that I did in regards to our first impression about Romeo and Juliet.
As a whole, my class and I decided that we felt that Romeo and Juliet were very immature. They are teenagers, however, who have probably never been in a serious relationship at such a young age. We felt that Romeo and Juliet were infatuated with each other, and did not really love each other. They didn't have the time to get to know and grow with each other, like they should before deciding to get married.
Romeo seemed very fickle. He was "in love' with Rosaline, and then he was "in love" with Juliet in the next breath. He seemed like the "typical" teenage boy, liking one girl, and then hurriedly moving on to the next.
Juliet had the qualities of the stereotypical girl. She reflected more on her feelings and seemed to think more with her heart than her mind.