I haven't played the pro-mage plot yet. I believe you get another Cullen quest there and this may flesh everything out more dramatically. In my game our interaction was pretty much limited to flirting, supporting him through his withdrawal and smooching a lot. I don't think it's bad. I completely enjoyed his romance and found it soothing amid the drama of the main game but I think if you didn't know anything about his past from prior games, there's not much drama to sink your teeth into. It's a simple little love story, which is not a bad thing in itself.
Yeah, I didn't support the rebel mages but Templars. If you do support the rebel mages, you do get extra scenes with Cullen, but that quest line isn't necessary to enjoy the romance imo.
The romance arc by itself was really sweet. Maybe I just have the hots for tall, handsome Templar boys *sigh* who knows. :3
Even though I am pro-mage in opinion, I supported the Templars this time because (1) I spoiled myself the rebel mage storyline
(2) that Seeker Lucius guy was cray-cray and I kind of felt bad for some of the Templars who are genuinely moderates like Cullen was except they didn't have a place to go to like him.