I recently started using Auctiva myself a couple months ago. I love it. Here's my opinion: If you have hundreds or thousands of items to sell on eBay, then it might be worth getting an eBay store to keep your inventory in.
But if you don't list that many things at one time, it's easy to figure out if it's worth it.
Ebay store fee is $15.95 a month
Insertion fee starts at .03
final value fee's are 12% for the first $25 (smaller percent after that)
picture fees .15 each after the first free one
schedule fee .10
pretty template .10
I only sell about 50 things a month, and I figure Auctiva easily saves me about $40 in Ebay fee's (I use 5-6 pictures for each item, and always schedule them to start at a certain time using Auctiva.) Even if I had listed directly on Ebay instead of using Auctiva, I would be loosing a lot of money in the image fees alone.
Many things I sell in auction format with a buy it now option, so the final value fee's are only 8.75%, which saves me a ton of money too.
It's ultimately up to you. If you have enough merchandise to justify paying the $15.95 for an eBay store, and the other fee's, then it might be worth it.
Diane