I just did my first serious campaign with Mailchimp. My list is relatively small, but I sent some new images and a coupon for $10 off a purchase two weeks ago. I had an uptick in sales here and a surge of about 30 "likes" on my Facebook page and a couple of direct emails asking about artwork.
I had 44.8% of the emails opened (industry average for arts and artists is 28%), and my click percentage was 9% (average 3%). Clearly it did well based on the sector averages, but it may be a small measurement pool. Out of 603 original recipients I had one unsubscribe and about 20 invalid email addresses.
All in all, I think it was a success. As I send more emails, I'll of course generate more data and I really start to see if sales are positively impacted by a given campaign.