Yes it is secure in many cases, but, as jase states, you can get screwed!
Some tips (I know some may of these may seem very obvious):
1) buy from sellers with good reputations (not just positive feedback percentage, since some people get negatives for silly things - I saw one buyer who left THREE different sellers negative feedback because they supposedly "stole packing supplies from USPS..." OR Monte's negative which someone left because Monte was "impatient", but also by reading individual feedback comments.
Also check the feedback of the person who left the feedback for the seller.
This can make a big difference, in my mind at least.
2) ask for photos of the ACTUAL item (if not already posted) - this can not only show the condition of the product, but also show that the seller probably owns it
3) e-mail the seller to get a little feel of the person (optional)
4) NEVER pay by check or money order
5) if it is too good to be true, it probably is
6) research the item (see what similar items have sold for recently, don't trust the MSRP or opinion on quality of product provided by the seller, especially if it is an older or outdated product.
7) If paying by PayPal, pay by CC as opposed to them taking the funds from your PayPal balance, since you can dispute the charges with your CC bank (or do a chargeback with PayPal).
8 ) Trust your instincts (when it comes to negative thoughts at least); it is better to pay a bit more or wait a bit longer for another seller to sell the item, than to get scammed or be disappointed with a subpar product ("not as described").
Ebay is not my first choice by a long shot (I have yet to buy there actually), but there are certain items (older, outdated items, collectibles etc.) or great prices (someone who just needs money or wants to get rid of something quickly because they replaced it [upgraded]) that almost exclusively be found on ebay.
EDIT: Perfect example of #7 is to read jase's post...
Plextor PX-716A TLA0304
Plextor PX-716A same TLA
LiteOn 52246S 52X CD-RW
LiteOn 52246S (another)
LiteOn 52327S 52X CD-RW
TDK 40X USB 2.0 CD-RW
TEAC CD-W540E 40X CD-RW