Hi,
I've been on Auctiva since May, and love it. I decided that when I started, I wouldn't try to use everything. I too am experienced on eBay, but A. was new. I don't have a store on Ebay - I'm just one of the little guys.
But I use Auctiva for many things at this point. I create all the listings, take advantage of the templates, use it for the multiple images, and schedule my auctions. I still use eBay checkout and haven't ventured into the Auctiva insurance realm yet, but I probably will at some point. I also use Auctiva to automatically post positive feedback once I receive it. That works nicely too, and saves me time.
I still prefer My Ebay page over Auctiva's closed sales page. I feel I can more easily at a glance see the status of everything. Auctiva is a little more limited, if you are not using it's checkout feature. I also like being able to add notes in My Ebay on individual transactions. So My Ebay page keeps me pretty organized.
Customer support at Auctiva is awesome, so if you have any problems you can always file a support case actually have a human conversation with someone who cares - a very nice feature.
Hopefully my experience is helpful in some way.
Wanda