I think I may be in a minority of one...
...but I really did not care for Sam Stewart. She irked me to no end and I found the writing surrounding her character was so lazy. And it started with the first episode. So they tell Foyle his driver's name is Sam and surprise!!!! Sam is a woman! Then, later on he goes to meet a man, leaving her in the car, and when it comes time that the man starts running away, Sam is standing directly in the path that the man is running, holding a garbage can top and stops him from getting away. There is no way she could have done that, even if she had followed Foyle. And while, in and of itself that is no big deal, that just seems to be her role throughout the show. Always, always, in the exact perfect spot to move the story along. Her character was almost always connected to every event or person involved in any of the crimes that Foyle was investigating. How many times was she working in a place that was the scene of a crime, or friends with someone who had vital information, or positioned to hear a very secret conversation. Foyle didn't even need a police force, she was connected to every one and everything.
In theory, I think her character was great. Having a woman who could do and say things that the cops couldn't would be very useful. I just felt they handled it poorly. Sam Stewart was absolutely terrible at undercover work. The two times she was supposed to be undercover she was exposed quickly for asking too many questions. There was even a time where she wasn't investigating and she still caused trouble and got someone killed. (And what was with sending her undercover using her real name? They did this with Milner too. But she knew way too many people for her name to not be recognized). The later episodes, I felt were just as bad (although I haven't seen the last 3, they aren't available on Netflix). Posing nude just because the man wanted her to? Using Foyle's name to conduct an illegal search. Seriously? And again, the fact that her husband would get involved in things that were directly connectable to something Foyle was investigating is what I mean by lazy writing. It was as if they never had to try to hard to create a story or a mystery because Sam Stewart would be right there or know (or meet) the right person/people and the once she communicated with Foyle then all would be resolved.
And I really don't understand the way romance was handled in this series. Again, for me this was lazy writing, and I don't think I have seen it done in British TV before. It was like, for added drama: just add a Sam Stewart love story. She got three proposals (and was also involved in three explosions), that's quite a lot. Apparently no man that was not part of the police force could resist her, though she did manage to get Milner into trouble with his wife once. I think the worst was her relationship with Andrew. Andrew, who was in a serious, but secret relationship with Violet (I think her name was) he then calls it off just because. Then Foyle convinces Sam to go out with Andrew, and the roller coaster of their relationship begins. It seems to me that the relationship was so unsteady because the actor wasn't always available so they had to write accordingly. But there really was no reason for them to have gotten involved. The whole show in MY opinion, could have done without their romance.
Anyway, like I said, I may be in a minority of one because the only bad things I've come across about her character are the silly things they have her doing in later episodes (some I mentioned above). And I expect to be disagreed with, but she just kind of ruined a mostly good show for me. And I really think in the later episodes, they focused on her so much, that it should have been renamed Foyle's War; Stewart's Life. Just my two cents.