We've stayed in Koca Ojstrica in late June (my family, our dog and my mum).
I had this cottage in my mind since first (day) visit to Velika Planina and decided back than that I want to stay here longer. Temperatures in valley/cities were above 30◦C (86◦F), so we literally escaped from the city heat to beautiful mild summer temperatures (+20◦C). There were few chill mornings after one rainy night, so we turned on underfloor heating (yeah, such a luxury and comfort!). We didn't used great-looking sauna, but it is on "to do list" for next time. My mum slept in big bedroom, and my family in the attic bedroom. Such a fun for kiddo! My balance was a bit rusting climbing up the ladder at first, but the next day I could do it blindfolded :-)
Since smart packing is really recommended, we used breakfast service (if you prefer home-made yogurt, cheese, bread, ...) and planned to use what is available in cottage’s self-store (yeah, another luxury!). Instead of letting bread being brought to us, we went every morning for to shepherd's cottage (Pastirska koča pri Gradišekovih) for a walk and pick up bread. Weekend mornings they even serve fresh baked "flancat" (type of fried pastry). My kid loved it, we had to go two days in a row there for his breakfast. We really enjoyed food made by shepherds and Slovenian traditional food in restaurant "Zeleni rob", so we didn't have really a chance to cook by ourself (although kitchen is perfect). The ‘shop’ has almost everything you need to cook a decent meal.
My recommendation: stay at least 3 nights, especially if you are a hiker or just prefer to enjoy amazing landscape and beautiful view from this cottage. It is so beautiful, that we even had some day-visitors coming to terrace. People came for a photo-shooting, while a cow, well, probably came just for sweet grass :-)
Host's (Rok) hospitality is beyond words, really. I've never encountered such an excellent service before. Better than “Hotel Mama”, I say! Not to mention, that even our dog was treated like royal: got its own bed!
We will definitely will come back again. I prefer summer time, although winter time might be magical as well…