Assuming they were kept in a decent state, and had ANPR on them so you didn't have to stop and fish around for change or a card, then I'm all for them. I always use the M6 Toll when heading out that way, and you only need look at France or the US to see how toll roads can be done properly. Since they'll be something that you don't have to use if you don't want to, I can't see any issues with this plan at all.
Yes, it will need careful overseeing to ensure it doesn't end up like Bronzee's examples, but it doesn't automatically have to end in disaster.