I use Fidelity and also use their amex card. I mostly stick to the Spartan SP500 mutual fund and have not gotten into ETFs. At least for most the Fidelity mutual funds that have no transaction fee, they require a $2,500 minimum initial purchase and a minimum balance of $2,000 in the event the fund drops in value.
Also, if you want to invest elsewhere, there are no restrictions on transferring your cash back reward out of your Fidelity account as soon as it arrives. I do this often when I haven't maxed out my Roth IRA, which is not held with Fidelity.