Cazoomi Syncapps talks to our CMS/Ticketing program and connects it with Mailchimp, which is pretty important to us. When it works, it works well. It's overall pretty inexpensive
It frequently doesn't work. I've had to go into the back end and reset the sync process multiple times, pricing changes sometimes without the ability to get back an old price, and errors can live in the system without any support for months.
Syncing our database with our email client - it does that successfully (in general), but frequently doesn't work.