I use power market and there has been zero hassle. I signed up last year, have been using their service all year. The only downside is they do bill directly from a checking or savings account. With CMP I can pay them with a credit card. I can't do that with power market but thankfully for me it's not an issue.

As far as savings. I've saved close to $300 this year alone so it's been worth it. My bill for CMP each month is $13.73 and power market invoices me by email so I know how much will be debited for my checking account. It's about as hassle free as it can get.