Paypal is a safe, easy way to send and recieve money over the internet keeping your bank details hidden from the third party. This is what Paypal and Ebay claim anyway. Since Ebay own and operate Paypal they take every oppertunity to push the service almost forcing it on you as the preffered method of payment on the hugely popular aucton site.

It does keep your bank account details hidden. - Your account details are known to Paypal who can withdraw funds directly from your account should your Paypal balace not be enough to cover any transactions.
It is also very secure - a lot of effort has been put into verifying your identity and keeping your details safe, a password entry to the site is your first line of security. They will also credit your account with a penny and ask you to verify the transaction no. to ensure you have full access to a valid account. Another security step is they will take your phone number and an automated dialer will phone you with a pin no. which you must recite back again to help verify you are the account holder.
These measures are the most comprehensive I've seen online and should ensure a secure environment for your online transactions.
It is quick and easy - After making a purchase you can have it paid for straight away and hopefully have your purchase winging it's way to you without delay. You can pay for goods or services or even send money as a gift to someone instantly

Paypal claim that you have no risk making a purchase since they will reimburse you in the event that a seller takes your money but does not deliver any goods. This is not strictly true, they will instantly reverse any charges they have taken off you and try to reverse whatever funds have gone into the sellers account however if they have no funds in their paypal account and they sever their bank account from their Paypal account then you will not get your money back. Paypal will open a dispute and a claim but they have no way of reimbursing you from the seller and they will not cover this loss to you themselves so don't fall for believing your transaction is totally safe - you can still get seen off. This happened to me once, fortunatley only for £18 but the outcome would have been the same if the amount had been more, I never saw it again and all Paypal do is leave a dispute open for years because the seller did not have any funds in his Paypal account, he withdrew eveything to his bank account then withdrew it all and closed it leaving Paypal with no means to claw it back. From what I can gather a lot of people lost similar amounts to this seller.
Personally I have a small debit account purely for online purchases which I use that only ever has a couple of hunded pounds in it at any time. I figure if anyone gets hold of my details all I will lose is £200 max.
Backing the Paypal account up with a credit card would protect you if you are seen off as it becomes the credit card companies responibility but do you risk someone getting hold of your credit card details, I'd rather not.

Customer support always there if you need it - Live help is available online with minimal delay, telephone help is also there but a spell on hold is a possibility.
Also be aware costs can add up - You are charged for everything, you have to pay a nominal amount for every transaction you make, especially if you are going through Ebay, you have to pay listing fees, relisting, selling total fees, if you are sending money the receiver has to lose a small percentage unless you send it as a gift in which case you cover the costs.

In summary Payal is a handy way to pay for goods and services if you can live with the charges and efforts have been made with security but it's not as foolproof as they would have you believe and there is still the possibility you could be seen off despite claims to the contrary.