I liked him from the moment I met him, though it took me quite a while to decide to romance him--I'm a lesbian IRL and I had a stupid hangup about how I should show my appreciation for the inclusion of same-sex romances by choosing one. Stupid I know, but ultimately I had to admit that Thom was a much better romantic fit for my Inquisitor than either Sera or Josephine (neither of whom really made sense as a romantic partner for her, though she was friends with both of them).
I think way back when, I did see some comments on the forums from players who found him boring. I never thought he was, but for those who did, I expect Revelations was quite a surprise. 
Also, I really like how his relationships with other members of the Inner Circle change after the truth comes out. I had him and Dorian together in my party a lot because I'm really fond of both of them, and I was always sad at how they constantly quarreled. When they made up, I almost could've cried it made me so happy. (Yeah, I'm a sap. Whatever.) And Varric was kind of snippy with him earlier on, and becomes friendlier after Revelations. I can't remember exactly how the banter went, but Varric says something about having been too hard on Blackwall, and Blackwall is like, "So you don't think I'm dreadful?" and Varric says something like, "I thought you were boring before. We're all dreadful." Oh, and he and Cole get along a lot better, too. The biggest scare I got while romancing Thom was wandering around the Western Approach before Revelations with Cole in the party. Cole started to give what, in retrospect, was an account of how Thom had had Callier's children killed. But I didn't know what was going on at the time, and it really spooked me. Of course it was clear there was something in his past that was gnawing at him, but I didn't know what yet, and when I heard that, I was like, "Maker's balls, what did you do? What if it turns out to be something I can't forgive?"
On a related note, having very recently replayed Revelations, I was reminded of the fact that Sera's comments on Thom change if you talk to her during Revelations, and change again afterward. That's some attention to detail!
Well, that got a bit rambly. I really like companion banter. Can you tell?