I want to start by saying this is NOT a budget phone. It may be in the budget price range, but don't let that deceive you. This phone has a crisp colorful 5 inch HD IPs LCD screen. Text is sharp and without blur or distortion. Photos and video viewed on the screen is clear and colors or very true to life. The clear black panel insures good outdoor use and reproduces blacks dark and inky. The one drawback may be the viewing angle is not as great as some 1080p or higher amoled screens. Although I didn't find this to be a deal breaker.
Build quality is miles better than other phones in this price range and honestly even bests phones twice the price. Microsoft smartly stayed with tried and true Nokia design team and the phone feels and looks just like other phones in the Lumia line. Because the entire back panel is removable to get to the battery/sim/micro SD area it means the phone body is free of creak inducing seams. So this means the phone has a quality unibody feel and look. The back panel also can be taken off to replace with other color panels to fit your mood or just to replace if your original one becomes damaged or worn.
The main camera on the 640 is an 8 megapixel shooter, and I found it more than capable of taking pictures daily. It wont supplant your pro camera or the likes of the Lumia 1020 or like, but then it's not meant to. The 640 comes with Lumia camera as ita default and it is a rich and full camera app that lets you customize almost everything down to white balance and shutter speed. I found pictures taken to be of very good quality and detail, especially in well lit areas. The night or dark shooting was adequate but isn't the best on the market, although at this price was definitely better than most.
This phone has a 1.2 ghz snapdragon 400 with 1 gig of ram under the hood. While that might not be overly beefy or blow most away, because of Windows 8.1 optimization it is more than fast enough.