Tim,
It's easy to sign up. Go to
www.paypal.com. I am not full blown user as I don't have it linked to any bank account nor do I accept money through it as I am typically not a seller. The 3% most people reference is the vig/fee that paypal charges a seller for their service. Some people evidently think that paypal should offer their service for free and ask that you pay by "Paypal Gift" so they can avoid the fees on the receiving end.
I use it to pay with and I am a confirmed user as I have it linked to a credit card. From a seller's point of view it seems to offer almost instant access to your money as opposed to waiting for a check to clear, etc.
As a buyer it allows me to use a credit card to pay for things and not have to exchange any credit card information with the seller. It is very convenient from a buyer's POV. I cannot speak to it as a seller.
It is very easy to use as a buyer/payer. On ebay auctions you can click the "Pay Now" button and it will take you directly to the PP website where you log on and pay to the seller's account/e=mail address. For a non-ebay transaction like off the BST you simply enter in their paypal info, select the method of payment (out of your paypal account if you have a balance or a linked credit card), click and confirm. Very easy.
Jeff