Obviously, there are some advantages to using a pre-configured device. They should just be plug-n-play enabled and optimized for that one service provider. Very little fuss on the user's part, as far as setup goes.
The down side is that you don't have any flexibility of configuration or mobility between service providers.
BYOD service providers have opened up their networks to allow you, the user, to decide what soft-phone or ATA (analog telephone adapter) to use with their network. In which case, you decide what model of soft-phone, or ATA, to use and in what price range to buy. The down side to BYOD is that, typically, the user is required to have a little technical savvy to configure their own devices with whatever service providers they choose that support BYOD.
I personally subscribe to Voip.ms and CallCentric as my primary service providers and have tested them with most of the configurable softphones that I listed in: “User Configurable Soft-Phones".
As far as my BYOD hardware equipment, I connect to CallCentric and Voip.ms (as well as some other BYOD VoIP services) using various VoIP adapters such as Cisco/Linksys PAP2T, SPA2102, SPA112, SPA122, SPA301, SPA504G, and a Grandstream DP715 DECT IP Phone.
In the end, BYOD service providers tend to cater to the more technically inclined, but at a lower cost base than a full service provider. So, with BYOD you potentially save more money and benefit from greater flexibility to jump from provider to provider - depending on your personal needs.