Just bought a new MacBook Pro 12.1, early 2015 (Yosemite 10.10.4). I'm observing memory usage from
kernel_task varying between 0.8-2.0 GB. Is it normal? Couldn't find a question referring to normal values.
Yes, that amount is normal.
As kernel_task performs a lot of tasks on behalf of user-space processes (i.e. the ordinary programs you're running on your computer), the actual memory usage of kernel_task will vary a lot according to which programs you're currently running, and what you're doing with them.