This was one of the best things I did in Vietnam. I did a two day trek with a home stay between, and I saw a side of Vietnam that would otherwise have been inaccessible.
The trekking was really enjoyable. I was taken by a local Hmong guide through local villages, and had a great view of the rice paddies, villages, and rivers around Sapa. There were plenty of good photo opportunities!
However, what differentiates Trekking Sapa from other tour companies is the human experience, which was truly incredible. My guide was a local farmer, and took the opportunity during our trekking to point out agricultural methods, teach me about local plants and foods, and allowed me to (hesitantly) feed a buffalo too! During the breaks from our trekking, I was also fortunate enough to prepare and eat food with locals, and had the opportunity to learn about how they lived, made their clothes, and farmed. I also stayed at her house at night, and enjoyed a really nice evening around the open fire, drinking rice wine and learning the Hmong language.
I would definitely recommend this to anyone who comes to Vietnam!