I recently built one with my raspberry pi and the camera module. It was stupidly easy, all the difficult parts (motion detection, saving short video recordings) are all done for you.
I upload everything to s3, and set notifications on the bucket to my phone. If motion is detected, I get an email within a few seconds.
If you're serious about not being spied on, the only reasonable thing to do is to build it yourself. Luckily, these days, that's very easy and cheap.