If you MUST have engraving, the Apple online store free service is certainly a good deal. However, as others point out, I would recommend NOT getting any engraving on your devices.
The resale value is drastically reduced. Check with Amazon, the "sell your mac" type sites, etc. and you will find that for an iPad with engraving the resale can be up to $100 or so less.
I had my iPad2 engraved, and never again. If you must have your name or something on your iPad, use a sticker or some type of label.
When we get a nice shiny new iDevice, we tend to think we will have it "forever". But after two short years we are ready for a better model. Engraving will prevent you from selling your old one for a reasonable price.