One thing to watch out for is that it does not have as many 4G bands supported as the 650, so (since you mentioned 4G in your first post) if 4G coverage is important I would recommend the 650 instead of the 550.
I got a 550 which my mom uses and from whatever occasional times I use the device it seems to be good. It's still on "current" 10586.494 build and it's not super-buttery-smooth but it's pretty darn good.
Mostly just use the phone and few apps like Outlook for email, Facebook, Whatsapp, some Edge web browsing... all on WiFi and are fine ... but again... we're not using the device for anything "heavy" like games or lot of web browsing. Pretty much hardly/never used it for photos/video so far.