Unlocked GSM phones work just fine on Primus. You could even use an iPhone on the network. When you sign up, they seem to require that you buy one of their basic phones, but after that, you're free to slip the SIM card into the phone of your choosing. Right now I'm using a Nokia N82, mainly for the excellent 5 Mpx camera (the low-end phones that Primus sells only have VGA cameras). Primus doesn't offer any real data plans (1 MB of data for $7, via slooowww GPRS), but I can use the N82's WiFi connection at many places to transfer emails, pictures, news, etc.

It would be great to see Primus offer some real data plans, but unless they bump up the speed to at least 2G, there's no real point to it. Possibly their contract with Rogers doesn't allow them to offer data services.