I did this journey (Krakow-Wroclaw) with my missus 2 years ago as we had a trip over there for my birthday. We went on the (not that direct) PKP IC loco hauled service which runs via Czestochowa and Opole. I booked the tickets in advance and printed at home, it was about £16 each for 1st class with reserved seats. The first half of the journey was fairly slow and the track was awful, must be one of the bounciest rides I've experienced, although the second half of the journey was much better (and faster). I think the whole journey was about 3 1/2 hours.
1st class was fairly comfortable, standard european coach with compartments and 6 seats in each. We ended up sharing a compartment with 2 very friendly Polish ladies as that's where our reserved seats were but most of the other compartments were empty. Standard class was very busy with very few spare seats, and I walked the entire length of the train at one point to get some drinks from the buffet only to find it was closed!
An enjoyable journey as it was my first experience of Poland, if I went back (on my own) I'd do the same journey over a couple of days and stop off at some of the busy freight spots and the depots on route.
My advice would be book in advance, 1st class and reserved seats, and stock up with supplies from one of the many shops at Krakow station before you leave.