I like the Sales Events Tool! Especially how it put the sale price at the top of the listing and how it calculates the percentages and shows the "sale" and "was" prices!
I like the possibility of being able to automatically post my Etsy listings on my Facebook page, so even when I'm busy and I'm not at the computer Seller Tools does the job for me. I have increased a lot my shop views and sales thanks to Seller Tools!
I tested it by using it, then not using it. The "not" using it was hard. I found it much easier to schedule the posts, then forget about them. I did wish that I could have put more specific information for Etsy teams on Twitter so they would be caught by them better. but the pictures worked extremely well. I found I expanded my target audience using the system, and sold more, which is the bottom line.