Personally, I do not think an SSD is a good storage medium for Time Machine because like @Simon said, TM is throttled and does not take advantage of the write speed of the SSD. However, SSD goes really well with Carbon Copy Cloner - my typically backup operation takes 3-5 minutes. This is really advantageous when I need to leave my desk frequently throughout the day, for days at a time.
I find many general points made in this discussion thread about SSDs and HDDs informative and helpful: