Yes, it is. The network is most of the times working well. You can also call to europe/america in good conditions.
Yes, there are several internet cafes in Kumasi. Most of them have good connections. For one hour you pay around 1cedi (50eurocent).
You can bring your own phone and buy a prepaid card (mtn/zain) for a low-cost in Ghana.
With these cards you can make cheap calls within Ghana and also to Europe/America/...
If you bring your own phone, you are responsible to take care of thieves.