I like detective games, and this one was pretty good, even if it was a bit short.
Only things I would suggest was that there should be more interaction with the suspects, like questioning them about any evidence you've found and letting them defend themselves if you bring up other suspects' testimonies. The other is that there should be certain things that must be interacted with to be able to arrest the criminal. (When I got the perp, Biscuit brought up evidence that I had no idea about.)
3.5 for this, it was pretty good for a first detective game. I'd like to see more of Detective Sir Biscuit in the future, it looks like it can be an awesome game series.