is somewhat undermined by the calendar that pops up for transitions between days
I still haven't read enough to form a strong opinion on it, though I definitely picked a good time for it, as a means to unwind from the consistently heavy mood of Fate/Stay Night. That said, I'm still in the common route, and I'm already feeling like I want things to get a bit more serious. I might have to start another VN in parallel, just to balance out the moods.
Things do get a little more serious towards the end of the common route but as Eclipsed said, for the most part it stays light-hearted.