Busy day today, so I'm just now getting around to this!:
When did you first meet Cullen (e.g. as Amell/Surana, in the Broken Circle quest, or in DAII)?
My first playthrough was with a Dalish rogue, so during the Broken Circle quest.
What was your first impression of Cullen?
I can't really remember my first impression of him, sadly. I do remember feeling mostly pity, mixed with some annoyance and frustration at his comments, though.
When did your impression of Cullen change (if ever)?
I was very surprised when he showed up in DA2. My first Hawke was a (pro mage) rogue, and Bethany ended up going with the Wardens, so I didn't see the part where he takes Bethany to the Circle. He was a returning NPC, though, and I felt a slight need to pay some extra attention to him because of that. My Hawke would drop by every now and then to chat with him, and he seemed polite enough, even if I really didn't agree with the things he had to say about mages. I started cheering him on a bit when he began to question Meredith. When the final battle came, I was sure that my Hawke was going to have to fight both Meredith and Cullen, but he surprised me and went against her. I actually felt a little proud then.
I'm really interested in seeing how his character further develops.