I was excited when Canon introduced the XC-10. Small, 4K, 1" sensor — all pretty good with the exception of the audio inputs. High bitrate recording aside the XC-10 was basically a DSLR with worse image quality.
Enter the XC-15. Canon has answered the single problem plaguing this camera with an XLR audio input adaptor that is exactly the same as the one on the C300. Now it is possible to record balanced audio and 4k video at a very high quality.
This camera doesn't take the place of the C300 model, but it does make for a suitable B-Cam and also a great event camera. While the smaller sensor will result in slightly reduced low light performance and a greater depth of field leaving some of your background in focus, this could be a great camera for those not shooting in extreme low light or cinematic situations.
One way I would use this would be for sporting events. The additional depth of field would be handy in keeping moving objects in focus. While shallow depth of field is nice for interviews, it is sometimes a detriment.
Also, despite the arguments against depth of field and sensor size, the trade-off in price and size might be worth paying the price. Plus there are other ways to achieve a shallow depth of field.