Pros:
1.) They don't charge a setup fee plus there is no monthly fee so if you don't sell anything there are no fees to pay.
2.) As mentioned, it's accepted by 100 million people.
3.) Integrated with eBay so it makes eBay selling extremely easy.
4.) Integrated with the USPS so printing shipping labels is extremely easy as well.

Cons:
1.) They charge a higher 'per transaction' fee for each sale made.
2.) Their 'Discount Fee', which is the % of each sale that they charge is higher than a normal merchant account.
3.) Very tough on chargebacks. If someone disputes a charge or fee paid, PayPal is extremely tough as the side with the buyer, deduct the money from your account, and then contact you. Many times charging a service fee of $10 for chargebacks.

I personally started with PayPal years ago and it was perfect for me as I was extremely limited with the money that I had available. I eventually moved on to true merchant accounts but I still accept PayPal on every single site that I run because of the amount of users. Click Here to learn more about PayPal.

Pros:
1.) Probably the best thing about 2CheckOut.com is that you can create 'Check Out' buttons for your site by simply placing some HTML code that is provided to you by 2Checkout.com. This allows you to add the button anywhere on your site without even having any Shopping Cart Software.
2.) 2Checkout.com is a widely recognized payment option so basically any Shopping Cart Software that you choose is integrated with 2Checkout.com.
3.) Because of the recognition of 2Checkout.com you're unlikely to lose visitors on your site because 2Checkout.com is a trusted shopping cart option.

Cons:
1.) They charge a higher 'per transaction' fee for each sale made.
2.) Their 'Discount Fee', which is the % of each sale that they charge is higher than a normal merchant account.
3.) The payment is bi-weekly so when you make a sale it's possible that the money isn't deposited into your account for 2 weeks.

As I mentioned 2Checkout.com is anther great low cost option for accepting payments on your website. Click Here to learn more about 2CheckOut.com.

Pros:
1.) Get paid fast! You'll receive your payments within 24 - 72 hours.
2.) Lower per transaction & discount fees. Generally more than 50% less so it means you keep more of your money!
3.) Credibility. When you have a true merchant account it builds credabilty because it means that you've gone through steps to prove your company is legit.

Cons:
1.) Monthly fees. Generally $20 - $25 per month for statement fees.
2.) Additional fees. You'll be required to have an SSL installed in order to accept credit cards with a merchant account.