My £0.02:
I just ended a contract with Pipex, and switched to Demon. Pipex were great, but too expensive. When I cancelled the contract (after the 12month period had expired), they charged me almost £200 pounds without warning or explanation. I have yet to get it back after 10 weeks, and they have been singularly awful at replying to emails and discussing this over the phone. I'm now having to pursue them through the independent arbitrators.
As much as can be expected, Demon have been helpful in the matter, and the Internet provision with them is just as good, but almost half the price I was paying with Pipex
Moral of the story:
1)I advise against going with Pipex
2)Pay by direct debit - this gives YOU far more control at the end: When you cancel your contract, cancel the direct debit, and instruct your bank that they shouldn't allow the ISP to take money from your account without your explicit authorisation.
Also, bandwidth limited services should be a last resort...
EDIT: Both Demon and Pipex supposedly offer support for Mac Users, but I've never had a need to test this. I've always found it more reliable to use a router with a modem built in than a USB modem, especially on a Mac. Takes all the business of connecting to the net out of the hands of the Mac / PC