We have 5 people traveling from Berlin to Krakow and want to know if this is a reasonable way to travel?
It's just a short ride, 5½-6 hours. As for quality of the roads, in both in Germany as well as in Poland you'll be riding top quality freeway practically all the way. They are basically equivalent to US Interstate Highways.
You don't even have to worry about snow or ice in the winter, these freeways are part of one of the main pan-European routes, they're considered priority one for deicing.
Keep in mind that the only reasonable option for parking in Kraków will be paid underground parking, which should cost you around 50zł (about $16) per day.
The distance isn't a problem, road conditions shouldn't be either (but don't expect US style superhighways) except some parts maybe in winter (snow and icing).
It's possible that there could be rental agencies disallowing entry in their vehicle into Poland due to some reports of car theft gangs operating out of there, but I seriously doubt it's much of an issue any longer. (It's not clear if it's still an issue, and most of the reporting on this problem suggested the cars were stolen outside Poland, then trucked there later for disassembly).
So there's no reason you shouldn't be able to do it.