Is there any sandbox to it, anything interesting to do when you're finished with the main story?
I cant really tell tbh cuz I finished it like 30 minutes ago ;)
As far as I can tell right now, I'd have to say "No". I haven't done every single little side mission yet, neither all races, so those kind of things you can still do ofc. Besides that there isn't much to do.
My biggest Negative right now would be that you can't get as "apeshit" as you can in the GTA-games. On the other hand, GTA 4 wasn't really inviting me to do that either, so...
The game shines at the fighting. It uses that Batman-system. Light, Heavy attacks and counters. Come to mind, there are fight clubs around the city which you can visit any given time, over and over again. I did those a lot
Fighting 15 triad brawlers at once is kinda cool
Overall, I'd say the game gives probably 30 hours of really good fun. I wasn't bored even once.
I'd suggest, if interested, to check out those keysellers. I bought my key from one of those for... I think around 30€. Fun fact: Game is on selling stop in Germany, buying the key from a seller gave me the opportunity to get the PEGI-version (Uncut!)...