I liked Lorne, but I'm gonna go with all the others and say there's really not too much to discuss with him until the surprise in the last episode of Angel.
You know, now that I mention it, I'm not sure that I liked Lorne. He had this annoying way of acting sugary and sweet while he was really being sort of nasty. He never really felt sincere to me. I felt like he was hiding his true thoughts and feelings underneath his eternal good cheer. I mean, would he really have been tolerant of man and demon alike if man and demon alike hadn't shelled out the bucks at his club? Would he really have befriended Angel and co. if they hadn't had a room for him in that hotel of theirs? And would he have been nice to Fred, Cordy, Angel, and all the rest if they hadn't just happened to be a formidable fighting force that were also easy on the eyes (something that seemed rather important to Lorne)? Eventually, his actions and his long-time support of the group led me to believe he was more sweet and kind than nasty, but there was a superficiality to him that bothered me throughout the series. Add that to his lack of any compelling personal arc and/or growth and the limited backstory that we have on him and, well, what else is there to say about him? In the end, we never got to know him very well. He always seemed to be putting on a front.
I felt shocked and disappointed at the end of Angel. I hadn't expected that of Lorne. I had believed that, no matter what, he was at least non-violent. The more I thought about it, though, the more I wondered if that was really true. He had, after all, for several years used his mind reading abilities to assist Angel and co. in weeding out the bad guys. All of that had worked to his considerable advantage, as he had landed a sweet PR gig for a giant L.A. corporation. Sure, he'd never gotten his hands dirty, but he was certainly complicit. Lorne was a performer and a people pleaser. What of he'd had us all fooled -- all of us but Angel, a "good" demon himself, who knew that Lorne really did have enough of the demon left in him not only to kill, but even to fool the intended victim into trusting him up until the very last moment?
There's something fishy about Lorne. The trouble is that the show never really goes into it in much detail, and so I think that's why most BBers take him at face value and don't discuss him very much.