Depends on shield... I use Steel Shield because:
+ style
+ resistance (I wouldn't use it, if i hadn't 9 shield skill, which raises resistance by 10%/skill point)
+ quite nice hp (maybe not best about guys with big axes, but it's perfect counter to people with weapons WITHOUT bonus against shields)
+ it's heavy - I don't care about bar mace and long iron mace, on the other hand people with heavy enough weapons (long maul, great maul) are too slow to get me - 9 athletics ftw.
There are few - :
- it's heavy. not only it slows me down in moving, but it's heavy and you feel it - you can't use it to feint, too low shield's speed. but shield skill and athletics speed up both moving speed and shield raising speed.
- to be used properly, you need high shield skill. otherwise you don't get anything more than usually - well, if you want to use it with 6 shield skill and 6 athletics, good bye. look prev point.