We got Mario Party 8 today. So far me and my brother have played around 3 hours. It's an awesome game. The minigames use the Wiimote very good. Although one thing i don't like is that when you hit the dice block it feels more like a delayed button press.
Also i don't like that the Single Player story mode, doesn't actually have a story, or a story mode that i know of. You play some thing called the Battle Star Arena, you basically play against 1 CPU player by going around a map they pick and collecting coins so you can get a star. It has an objective you have to do to win. Say like Collecting 50 coins and getting a star first, buying 2 stars first, etc.
I don't really notice any graphical improvements, although Mario Party isn't known for it's Extreme Grafix.