For that specific box, I’d keep it short and natural:
What I liked most was how easy it was to use, particularly as someone who knew very little about building. I was doing an extension on my house and it gave me a much better understanding of the quantities and costs involved. It saved me a lot of time getting my head around the job and, more importantly, helped me keep a handle on the costs. Review collected by and hosted on G2.com.
I’d give it a small, believable criticism rather than saying “nothing”:
There was a bit of a learning curve at the start, especially as I had no building background, but once I got the hang of it I found it very straightforward. A few more simple guides aimed at complete beginners would be useful, but otherwise I had very little to complain about. Review collected by and hosted on G2.com.