I've been using my Apple Watch Edition with Spark for the last couple of days and it's working well, and as expected. So far, all the apps that work on WiFi (no iPhone on) also behave the same way on cellular.
Note that if you are a Spark Digital customer you need to get them to add your One Number Plan for you and provide the Apple Watch EID and IMEI to them. After that is processed the plan is available and ready for the Apple Watch with no further setup. You do NOT use the 'Setup Mobile' process that shows in the Watch App > Settings > Mobile.