This is a wonderful simulator, anyone with an interest in space, astronomy, or just wants to watch mercury whiz around the sun should certainly consider picking this up.
The UI is cumbersome at first and it will take some getting used to, they have done well to hide most of the complexity slightly out of sight until you look for it which certainly helps ease you into it.
For what it does and what it offers, it's exactly what it claims to be and I can't help but commend the design team for it and recommend others to try it.
All negative reviews I've seen either had technical issues with it, or were clearly expecting something different. I also don't get why people keep saying it's not a game, it doesn't even claim to be one... doesn't mean you can't have fun in it.