I have a Z1 for the past 2 years... For most of that time, the camera has been unreliable. Then, they upgraded it recently to lollipop and hooray - camera fixed!
Unfortunately, with minimal apps loaded (and I even did a factory reset), the phone is now rendered so slow it's incredible. You can watch the icons draw. I'm not alone in this. There's a lot of people waiting for a fix - you'll frequently find yourself re-pressing buttons thinking you must have just missed them the first time round. For a jump into chrome, it might take >5 seconds for example. Come back out to the home screen, you'll randomly find a 5 second delay as the screen gradually fills in.
So, beautiful hardware - but what's the point if the software lets it down? For over a year, I was plagued by the camera popping up, seeing what you wanted in the viewfinder, pressing capture and then getting "camera not available". As a camera, it was totally worthless to me. Now it's just an unfeasibly slow brick.