My daughter started EPGY last summer when she was 4.75 and we've had a mixed experience with it--
Cons:
-The K level was too easy so she grew bored quickly-- they were willing to bump her up to first grade- also too easy, but I worried that starting her at 2nd grade would have been premature--had no way of really knowing; I wish it had been possible to customize the starting point in a more targeted way.

-Sessions include several "math races" that are designed to encourage speed, but my daughter hates these so they're a barrier to her wanting to do it--

- Although the program is supposed to promote the child quickly through skills they've mastered, it's not quick enough for my kid who's looking for a challenge and tends to be really excited about a specific skill (e.g., division) rather than wanting to slog through things in order

Pros:
- Great practice for basic skills
-Encourages multiple ways of thinking about concepts **This is the greatest strength of the program in my point of view
- Sense of accomplishment from incremental progress

I pair EPGY with IXL-- on that site my DD can go after the specific topic she's really curious about and doesn't have to be sequential. I find it's a good balance. Hope that helps!