As of 12/2014, we have been using Nokia Lumia 521 phones for about 10 months. We are using them currently on the T-Mobile network. The phone were inexpensive at under $90 per phone delivered with no contract. I read that a couple of new phones were coming out to replace the 520/521. The phones in question are Nokia Lumia 635 (and a similar Lumia 630) as well as the Lumia 530.
The Lumia 635/630 phone at first blush didn't appear to be much better than the 521. The major differences include a larger screen at 4.5 inches vs 4 inches on the 521, longer battery life potentially faster 4G connectivity, faster 4-core Snapdragon 400 processor and Windows 8.1. The phone was selling for a significantly higher price than the 521. But it still has the shortcomings of the 521 had and even a new one: mainly there is no front facing camera, so using Skype is not going to be very pleasant, the camera that the phone does have, has no flash whatsoever. And the dedicated shutter release button for the camera is now gone.
The camera uses a new 4 core processor which is going to produce faster response times than the one on the 521. Do these minor improvements justify the significant increase in the street price? For a while I decided that they didn't. I planned on staying with the Nokia Lumia 521 for a while longer, maybe until Lumia 735 or 830 are available on the T-Mobile network. The Lumia 521's dedicated shutter release button for the camera is a semi-unique feature, the lack of which was a deal breaker for me.
But later I changed my stance I ordered the Lumia 635 and was impressed with it once I got it. And here is the review.