Absolutely love them together. They work really well as a couple. They are the 'ordinary' ones, the ones with no powers so they none of that 'oh you're stronger than me' angst. Dawn can open/close portals now, but in the worlds they travelled to, she needed Xander's skills to survive, so they work really well together. I also have no problem with the age difference or the fact he used to babysit her, because there was never anything between them. My favourite thing about their relationship is how Dawn pushed Xander towards therapy and getting help for all his issues, and supporting him through all the difficult times he had.
I'm not such a big fan of them being married and having a child when Dawn is only 25, but that might be because that seems incredibly young to me and I'd want Dawn to have a full life before settling down, but maybe she has had, with all the adventures and things she's seen in her life.