As simple as Facebook to use If you are a student. As easy to administrate as a Wordpress site.
Some customization options are not smooth but I'm sure they will get better soon. Nowadays, it's complicated to deploy a multi-site environment without a little coding help. D2L knows these issues and they're working hard to solve it asap.
You will find really easy to adapt BS to your needs and your users will rapidly join in. It has several integrations with external tools already running and setting up a new one is not a pain. With a very powerful documented API and working services, anything can be plugged in. Vendors are serious about the SLA's and uptime.
The flavour of the system is completely 2017-ish, far away from the java-application-like feeling of the usual LMSs. Being the new guy in town is not a deal for D2L, it performs very well. It also has a pack of apps for students and teachers that makes 100% possible not to touch a desktop computer to manage or consume the system.
As a custom approach, I would say that If Blackboard is a Mercedes and Moodle is a Chevrolet, D2L would be a Tesla.
D2L Brightspace is an affordable, fast, cloud and simple LMS perfect for little educational needs, but escalates very quickly for huge scenarios. As a consultant, Brightspace would be the first choice for most of our clients, no matter if they're looking for their first LMS or they want to move from their current one. It haves a conversion tool that can turn Moodle, Blackboard and others' courses into D2L format with a few clicks!