I found that Saiyuki was an enjoyable anime to watch for many reasons:
a) The plot was interesting, throwing awesome characters whom all have their own story to tell, filled with action and suspense. The concept of the story is well told in each episode and does extremely well in protraying characters struggle.
b) The characters are interesting to watch, their decisions and actions throughout the series as well as their humorous personalities.
c)The humour in the what they do and how they talk gives me endless joy watching it, with some of the best humour I've seen for awhile.
d)It has bad guys that are strong and totally awesome for once, normally they are screwed up people bent on world domination (not that there is anything wrong with that =P).
e)Fantastic fight scenes
I could go on, but that would probably bore you, but Saiyuki is one hell of a show that you SHOULD watch!