Yes, it's big, but of course there are benefits with it. You have more real-estate on a reasonably bright and sharp screen. The MIUI is done very well, and the battery is amazing. The only downside is that it feels very delicate so a cover is in order.
At the moment I've got it, I thought that I made a mistake, that it's too big, but after just a couple of minutes I've fallen in love with it.
The screen real-estate amazed me so my media experience was so immersive. The screen is plenty bright and pretty sharp since it's a 1080p on such a large screen. Although there are complaints on it's Wi-Fi not connecting, I had no experiences like that, it worked flawlessly.
The best thing about the phone must be his amazing battery life that has got me two days of moderate to heavy use. MIUI coupled with the great performance of the 650 is smooth and without stutter. It offers a lot of customization and if you don't mind the look of it, you won't regret it.
The 16mp camera is performs great in outdoor light, without oversharpening and good saturation, close to the actual colors, however the same cannot be said about the low-light shots where you have to be very careful with positioning according to lighting to be able to get decent shots. The same can be said about the video that looks really good in natural light but is very blurry in low-light especially with the lack of OIS, so that steady hands are necessary for decent video.
Another complaint with it is that its not very well built with small crackling when you slightly flex the phone, that leaves you with the cheap feeling in spite of the weight. All in all an amazing phone for the price.