Pros
Excellent support staffed by people who are always helpful and encouraging, promptly responding to complaints and questions. Tasks are described clearly with plenty of examples, sometimes bordering on the excessive and repetitive which in hindsight is actually good for reviewing one's own work. Working interface is flawless, generally fast to load and submit, with useful tools like bookmarklets and browser extensions.
Cons
Leveling system makes it hard for novices to advance: few tasks are accessible when you are just starting. I got lucky by coming in when there were real easy tasks at level zero, and managed to jump through upper levels in a couple of weeks, but then I saw dry spells where all of the tasks are for levels 1 and up which would easily discourage newcomers to continue.