Nice interface, smooth transitions and original way to set date. Can have a white (like pictures) or a black background. Free version comes with ads but very small, which is a plus. It has the original 3 theories, not several unknown others like some apps. Other than that it looks pretty cool, very nice use of color on the white background, I like it a lot! And it's clean, there's somewhat of a hidden menu on bottom of screen with simple iPhone icon (three dots).
Pretty hard to not understand these super simple functions!!!
Why 4 stars? -I haven't checked it with the actually theoretical equation. -The full version just lets one save dates, albeit it's more save slots than the competitor, why no save function? And if I did buy it, why not idk 99 saves? Or infinite saves? Seems silly to charge for.
But those are honestly the only reasons. It's a great app for what one is looking for in a biorhythm app.