I've been using it for a while and really enjoy them! Never had any issues whatsoever.
Had very negative experience with critsend.com! They have got system glitch couple of days ago and instead of trying to solve the problem they closed my account. Nicolas Toper gave me support but I would better get support from somebody else in their company, because this guy was behaving like he was already drunk... He said that he rather close my account then try to solve the problem and find out what went wrong with their service. Actually the problem was related to wrong e-mail address in FROM field, their service just put there email@example.com instead of my company e-mail address which just caused deletion of my mails by customers without reading... Now, Nicolas Toper just closed my account and as he said applied for refund. Not very good experience with customer support. I understand any service may have problems but I think people from critsend.com just do not care about their customers and it is easy for them to say you bye instead of trying to find out what is going with their service. Also Nico said that all their staff is on holidays for the next two weeks so technical support is impossible. So if you are sending mail through them on Christmas time be prepared that in case of any technical problems nobody will help you and if you complain then Mr. Nico just will close your account. I liked the reporting tool of critsend but even if you find really good service the customer support may kill any goodness!!!
Better deliverability then sendgrid... more reliable for our mission critical applications