I use synergy for normal desktop use and when I run two entirely dissimilar characters. Bearing in mine I play City of Heroes not WoW (got bored of the grind and lag). I find with two N52s and one mouse Synergy works well in this situation.

I use MultiBox when the characters are very similar as I can get layouts to work better together.

I suppose I could work my way through to use MultiBox all the time but Synergy does the job at the moment.