Well, I've had my 650 for 5 days now. I can't say enough good things about it. Keep in mind that my 920 had W10M installed which ran slower than WP 8.1. The difference in weight, size, build quality, speed and battery life make my 920 feel a few years older than it really is. The 650 is very snappy to respond to touch and performs most tasks faster than the 920. I've read some places that going 920->650 is more of a "sidegrade" than an upgrade, which I was initially worried about. But I can confidently say that the 650 is definitely an upgrade from the 920.
The only feature that I don't have, which the 920 had, is the wireless charging. However, I stopped using wireless charging about 6 months ago for a few reasons. For one, my wife kind of took over the wireless charging pad for her Lumia 830 because I started plugging in my phone sometimes. The reason I started plugging in my phone is because some night it just wouldn't charge properly. I would hear the phone's charging tone every couple of minutes like it wasn't making good contact, no matter how many times I re-centered the phone. Also, sometimes my cats would move it, either by walking by it, or playing with it. Either way, it would get moved enough to stop charging.
Another thing I was concerned about on the 650 was the lack of physical buttons. The back, Windows and search buttons are instead on-screen. I actually don't mind it at all. And I think it might be even less work for the battery this way because there are no extra buttons to light up or have to monitor for touches. The OS is quite smart in hiding the navigation bar when needed. When viewing fullscreen pics or videos in landscape mode, the nav bar hides. Touching the screen or exiting fullscreen video brings back the bar. You can also swipe into the bottom edge of the screen to show/hide the bar manually anytime.
While on the subject of the screen, it's an OLED, and it looks amazing! Especially in lower light conditions where you can truly see the blackness of blacks giving you that infinite contrast that only OLED screens can do. Don't get me wrong, the 920 had a great screen too, but this one looks sharper, brighter, more vibrant. Another advantage of OLED is the glance screen. I had the glance screen on my 920 set to only stay on for a time, and then shut off. This is because of the IPS LCD screen. On the 920, the entire screen's backlight has to come on at a low level, and the entire screen has to go black, except for the text (this is how LCDs work). This way is a big drain on the battery because you're using the entire screen, and backlighting it. The 650's OLED screen on the other hand is much more efficient. OLED screens light each pixel individually because each pixel is a light source. So for the glance screen, only the pixels that are actually displaying the text are "on", and dimly lit. This uses MUCH less battery. In fact, since I got the phone, I set my glance screen to "always on", and I'm quite impressed with the battery life.
Battery life. In short, twice as good as my 920. However, my battery life suffered since installing W10M on the 920. But even with WP 8.1, the 650 still beats the 920 by almost double. With average use of texting, e-mails and browsing, my 920 would be around 20% or less by 6PM. My 650 on the other hand, is still at 50-60% by 8PM. I could probably get 2 full work days out of the 650's battery with less usage. The 650 also charges up quicker than the 920 ever did. I should also mention that my 920 was a European model, so LTE never worked in Canada because of the different frequencies. LTE on the 650 works great and it's very fast. But, this again shines light on the battery life issue. The 920 not using LTE should be using less battery than the 650 does using LTE, but the 650 still last much much longer.
I really do recommend it, especially for the price. I could go on and on about everything about the 650 but I realize now that I'm rambling on quite a bit and this post is getting quite long lol