Ok, this is my first Windows Phone: I also own a Samsung Galaxy S4 and a Nexus 5, both of which are Android driven phones.
The 635 is a capable phone with a pleasant OS. It is somewhat speedy with a decent, though not impressive screen. I do see the "screen door" effect with some colors (reminds me of the old Bionic days) due to its lower resolution but it is bright and easily read in sunlight.
Speaker sound is passable and is rather shrill at high volume and becomes slightly distorted.
The microSD card slot and removable battery are welcome additions.
Now, where things can get a bit underwhelming is some software nuances and regrettably, lack of apps.
First the software and its functionality.
Tiles are fun and swiping is easy.Good predictive texting. However, I notice lags and slight hesitation. Mainly, an individual has to touch the mic button to activate Cortana and there is a slight delay like "waiting to set up." Why? Google Now is immediate and voice activated w/o touching a single icon. Cortana is not immediate and the wait is a few seconds.
Email does not load instantly and I have to wait a second or two for the full message to appear and download. Gmail, on the other hand, w/Android is instantaneous in its rendering and delivery.
The browser is god awful in this phone with few other choices to be found save for UC browser and Maxthon, which sometimes hang. Pages render terribly and some don't even load at all. However, I do know the Preview Release of 8.1 R 1 is due this week which may address that and I unlocked the phone to become a Dev phone.
And lastly, there are virtually no apps which can compete with Android and IOs. Most of my banking apps are missing and there are crippled versions of popular apps. Many popular apps such as Starbucks, McD's, Wendy's, BK, etc are not present. Flipboard is not present. Major newspapers, except for USA Today are not present.
The camera is mediocre since there is no flash. Selfies are almost impossible.
Now, with that being said, the $99 price tag makes this phone obtainable for many.
I do recommend this phone as a BASIC smartphone or as a secondary phone, although the likes of a more functional MOTO G make it a difficult competitor. Again, the OS is pleasant, but I feel I only get "halfa phone" with its limited functionality and lack of apps. But, if you are on a budget and just need the basics, this is fine!