We just spent the day in Zagreb before taking a bus to Zadar. I've been to Zagreb many times but only stayed over once. It's a good transit hub to the rest of Croatia. Some day I'd like to stay long enough to go to the Neanderthal museum near Zagreb. I'd also like to spend more time in the upper town.
Zagreb is very Austrian in feel whereas other parts of Croatia feel Italian (Istrian) or a mix of other things (Dalmatia).