This was our (my wife and I), first trip to Poland. Indeed apart from a visit to the Soviet Union many years ago and more recent "long weekend" city breaks in places such as Tallinn and Budapest, it was our first holiday proper in Central or Eastern Europe.
We were there for nearly three weeks. But we are quite lazy tourists and likely spend longer in a place than the specific sights necessarily merit, often we "waste" hours just sitting in parks, lingering over lunch/drinks or aimlessly ambling around streets rather than inside museums, cathedrals and so on. Others could accomplish what we did in less time, or see more.
The headline is that we really enjoyed Poland:-
It has much to see and a great variety of sights - mediaeval towns, early modern palaces, museums and memorials (the amount of the latter two partly reflecting perhaps its rather "up and down" history, especially partitions and then grimness in the 20th century).
We were fortunate with the weather. We chose August/September as being likely drier compared to October/November which is when we usually travel in southern Europe. That turned out to be a good (or lucky), decision.
Getting around by railway was straightforward and good value. Same with local city transport such as trams and buses.
Practically everyone we dealt with spoke English (certainly enough for our tourist purposes; many younger Poles are fluent).
Food was varied and tasty - tending to delicious "hearty fare" of filled dumplings, soups, thick meat cuts and sausages; though no shortage of modern, lighter cooking if that is your preference. Poles apparently love good ice cream, putting even the Italians to shame. It was on sale everywhere. There also seems to be a national mushroom obsession to equal Spain's olive fetish. There is an excellent beer scene, from the ubiquitous & refreshing cold lagers to good craft brews.
Any niggles were minor. Signage (Polish or English), could be better in places. We did seem to spend time at several sights walking around the building to find the entrance. And Poznan railway station must have been designed by a drunk (who else puts platform 4 next to platform 1?). Still, it all adds to the fun.
Before going I started reading (and finished whilst there), Adam Zamoyski's "Poland: A History". Very readable and informative - I had no idea Poland was such a big power in the middle ages and after, nor its quite early adoption of "modern" ideals long before a lot of the west (e.g. Poland had an equivalent of Magna Carta just a few years after England and had one of the widest parliamentary franchises of any continental country for several centuries).
Overall, it was a brilliant trip and we both greatly recommend Poland to others.