I've done both and still do both. I used one of the big online only agents for a few cruises and have never had an issue. Even though I'm in Vancouver, if I do use an real agent, she's on the East Coast of the US.
I've been lucky in that we've never run into issues on any of our trips so I haven't really had to weigh in on their services. Both have got me price reductions, found me better cabins and respond to e:mail very quickly.