I just got one a couple of days ago. I really like it. Picked it up at work after my shift for about $50.
I'm coming from a, still activated, HTC 8XT WP on Sprint. I loved WP on my 8XT, but got tired of being under contract and having a crappy selection of WP offerings from Sprint. Wasn't interested in continuing with Sprint for another year if they weren't going to treat WP customers better.
With that in mind, my 635 is my first Nokia phone, and it's my first prepaid phone. I've been on Sprint with the same line since 8th grade. I'm now 22 and the new phone is on a $45 AT&T Go Phone plan. Will soon save up for the early termination fee and drop my contract with Sprint.
As for the 635, I love it! I don't really miss the FFC as I rarely ever used the one on my 8XT. I also don't miss the ambient light sensor, or the better quality speaker as these weren't things that I used often enough to justify them. I just knew that I wanted an affordable smartphone on a prepaid plan. I also knew that I didn't want to go back to Android, and I never want to hassle with those stupid, and overpriced (in my opinion), iPhones. So, the 635 fits the bill perfectly.
It's small and slim enough to put in my pocket and not feel it or see it protruding through my pant leg. It's light weight and feels great in the hand. The screen is big enough and looks great for the price. The overall UI is very responsive, regardless of the low end hardware. And I can't get over having LTE in my hometown. Sprint doesn't have LTE coverage where I live, and only goes to 4G in the two nearby cities with my 8XT.
I don't like the small amount of built in storage, but WP apps are usually pretty small anyway, so I don't notice it. Plus, most of my apps are now installed on my 32GB SD card (will probably get a bigger one soon though as I've got the 32GB card so full of music). Call quality is terrific, and the audio quality through headphones is no worst than the 8XT (sorry Beats Audio, but you don't make a big enough difference for me to miss ya).
Regardless of the low end price of this phone, I don't feel like this phone is low end. If you're on a budget, like me, then this phone is definitely leagues above the Android offerings at a similar price.
If you want more features, however, then this isn't the phone for you. I think most of the those extra features touted by the more expensive phones are useless anyway, and aren't worth the extra price. But, that's up to you entirely.
If you can live without a few features, however, then this phone will probably be a good match for you. Not the best of the bunch. But, for the price, it holds its own.