I'm going to be honest with you, my first play through of ME2, I thought that Jack would turn out to be homosexual, but as I continued to play, her story developed and her romance scene with Shepard fit really well in the end. Although she's just not my kind of romance, I do think they did her relationship pretty well.
For one of my Male Shepard's I decided to go Miranda's way. I enjoyed the build up of the spark, and maybe the climax was suitable, but it did feel (to me) that it was more of a casual fling at the end, despite her wording and concern of "love".
Playing as a female Shepard, I had a very tough time with romancing anyone. Probably not due to story or choice, but just because I had no motivation to do it.