Tiger99,
Pay Pal doesn't give you an SSL cert, a Merchant Account, or a Gateway account. You get to borrow theirs. What this means is that when the customer is ready to check out, they are transferred to Pay Pal's web site for the checkout. Then, when they are done paying, they are returned to your web site. To an alert customer, this looks unprofessional. The average user might not notice that they left your web site. Either way, you lose some control over how the page looks and asks for the CC information. That's the main reason that people would still choose to pay for a Merchant account, Gateway account, and SSL cert.
Also, I have heard many nightmare horror stories about Pay Pal refusing to give people their money. I've heard stories about people who sell a product using Pay Pal, the seller ships the product, and when it's received, the buyer calls Pay Pal and says they never got it, the buyer gets their money back and keeps the product. (Which stings when it's a $2000 laptop). I've heard stories about Pay Pal freezing someone's account because they didn't like the product they sold (some sort of controversial product). Pay Pal kept their money.
See http://www.paypalsucks.com/
But other than that, yeah, Pay Pal is a great way to go.