I haven't tried it but I did try CloudBounce (which CDBaby is pushing) and eMastered. I liked how it immediately made my music sound better, but eventually I tried
mastering it myself and by doing some tweaking it eventually sounded better than that. I don't know... maybe if you don't know anything about this stuff and you're in a race against time to have people hear your music, maybe it makes sense? But if you're willing to put in the hours or know someone who is able to, it's always going to be better, I think.