The Cricket 650 is only locked for 6 months of usage on their network after purchase. After that, you would be free to unlock it and use it on compatible networks. I bought the 650 from Cricket, spending $120 versus the $200 for the dual SIM / unlocked version. Aside from Deezer and My Cricket, the rest of the applications are stock. We'll see when the Anniversary Update comes out in August if Cricket / AT&T does anything to expressly inhibit or allow the device to be upgraded. In terms of looks, the 650 is a nice looking Lumia. It performs well given the chosen hardware, but most Windows Phone users already know the OS does not tax processors much at all.
If you have a 550 and you are happy with the 550, wait until the Anniversary Update comes out and see if there is anything that states the update is available on Cricket. If the Anniversary Update is available, upgrading to the 650 for $120 isn't a bad option. Just keep in mind that they bill you ahead for a month's service when you upgrade online. In a store they will, at minimum, charge you a $25 activation fee. I got away without paying ahead for a month's worth of service. Just don't get the case that they have in stores, there are better options available on the internet.