Manulife One is a mortgage, HELOC, and spending account in one. So basically it's a fancy-pants way of saying "secured line of credit". You pay interest on it plus there's a monthly cost IIRC.
If I didn't already have a Visa through Scotiabank that ties in nicely with my regular accounts, I'd switch right away. If Weathsimple ends up offering comparable credit cards as well then I'll probably make the full time switch
Seems to me you would be losing out on cash back/credit card points. I guess if you can make bill payments from this it would make sense - then you could still use your regular credit card and make payments on that.
Or just use it as a high interest savings account.