I've got a pretty active DA account but have only been on for 4 years, so I'm sure my experience won't line up with the old timers.
I really haven't met any trolls on DA and I interact with hundreds of people per week. Spammers yes. Trolls? Maybe like three over four years?
Promoting webcomics, well I'm doing that, I can't say the audience on DA is super interested in webcomics. I'm using art as a feeder for the comic, that said, the people who are willing to give comics a chance usually give mine a chance.
Daily Deviation isn't something that brings in a lot of traffic, so though everyone really wants it, it's not as big a deal as you might think. I'll only get maybe 100 extra people coming per day if I get a dd.
Like any social site, you've got to build it. A long time ago I heard someone say that it takes an equal amount of time to go from 0 to 600 subs as it takes to go from 600 to 6000. I can confirm on both counts.
It's not got the greatest format for hosting comics, but I've made it work for me. Link navigation in the description works fine and the galleries are great for organization.
The DA site redesign Eclipse is a rolling disaster in progress which might cause you a lot of frustration in the short run because it's really unprofessional. I can't confirm whether new accounts are required to use it. If so, I'd hold off until they fix it, particularly because Eclipse makes new comics really hard to discover. I'm submitting bug reports to the staff weekly over this, and they've been relatively good about trying to go through the enormous bug list on Eclipse and address them so I have hope that someday they'll get around to making it good for comics. Everything's up in the air over this, and lack of certainty makes things difficult.