I had a lot of trouble distinguishing Sam's performance from Lucifer, so it's easier for me to think of them as seperate entities. I really enjoy Lucifer/Sam, I have to admit it. Dean/Castiel was such a success because the show finally had another guy to slash and it became a running theme of how much Castiel was sacrificing for Dean (his faith, then his brothers, and then his grace as an angel), which comes to an initial head early in S5 when he says, "I did it - all of it - for you!"

Yeah, real subtle there, writers.

As for Sam/Gabriel, I have issues there which have more to do with Gabriel's depiction because I grew up with films like the Prophecy and Christopher-mothereffin'-Walken as Gabriel. I think the fans were really excited to have a spare angel to give to Sam so he wouldn't be left on the sidelines of Dean/Castiel. And then maybe someone could save him (because if anybody was going to protect Sam from Lucifer, it'd be another archangel).