Sometimes, navigating around the product (it's cloud-based, as you would expect) can be a little slow and clunky. I don't like how you need to send an email for 'review' before you can send it. Sometimes this can take a while, but no more than an hour. While I get they are trying to protect the reputation of their servers, I'm sure they could employ a 'spaminess' checking feature. Actually, they do - Litmus - but you have to buy credits to use it, and I'm not sure it lets you bypass the 'review' stage.
Also, it seems they have changed the reporting. While it shows you how many emails were sent, opened, clicked, etc., it used to show Open Rate and Click-Thru-Rate as a percentages. It no longer does this. Strange, since these are industry-basic stats. Review collected by and hosted on G2.com.