I bought my Lumia 830 two weeks ago. Coming from a 520, I tested the 830 in store and was amazed by the efficiency of the device. Power, design, business yet playful look and a great camera for a great price (Lumia 830 is only 70 euros more than the 735 in the Netherlands).
Meanwhile, despite all the advantages, I've returned it. First of all the charger didn't work. That's not a major problem, since I have a lot of chargers and the USB-cable itself worked fine. Then after a few days, when I pushed on the back of the device, I noticed there was room between the cover and the battery. The pushing also made these kind of stains on the screen like when you touch a tft screen. That's a pity, because it made the device feel much cheaper, but still no deal breaker for me. Untill 2 days ago the screen came loose from its shell in the top right corner. The problems with the pushing on the back seemed to increase (maybe because of the disbalance in the device, with the loose screen). I returned it today. This is not the Nokia build I was expecting with my recent experiences with the 520 in mind.
Tomorrow I'll pick up another white 830. I'm not judging yet, maybe this was just a 'monday morning' device. My roommate has the black 830 and his is working great. Fingers crossed....