The 635 is a good device for a lot of people.
I got one for each of my grandparents to try their hands at smartphones. They're not that old (68 and 73, respectively) but get afraid they might break expensive things. For $100, they needn't worry. They're putting in their own contacts, making calendar appointments, and checking weather for upcoming trips--and they don't even have them on cell service yet!
I tried the 635 and rather like it. Won't become my daily driver, but it's definitely an upgrade to the 520. I'm keeping the 520 as my primary windows phone (I use an HTC One and Moto G ahead of it) until the 730 and 830 come out. I want to see what they bring to the table.
For a more basic smartphone user, or one who doesn't need cameras for selfies or personal photography, the 635 does quite well.
If you're thinking about it, make the jump. Well worth it.