Join Date: Jan 2012
Location: Cadian Battle Cruiser "Divine Wrath"
Overwatch only starts to become really useful and devastating with the divination BS-boosting in overwatch, when combined with a large blob of mans (say DA Libbies/Ezekiel with a 30man IG blob or even just massed low AP such as plasmavets or Combi-Sternguard, or even maybe a farseer with a crapton of guardians). That becomes the equivalent of an extra round of free full BS shooting, and with a high volume of shots (such as guard), that can be devastating to charges (particularly as the casualties are likely to rob momentum from the opponent).
In terms of CSM, i agree with previous posters. A few bolter shots are not THAT useful, but sometimes that one off fluke is great, but not to be relied on. And i dont believe that the Tau's supporting fire /Counterfire is that exceptionally important, given that it may only map to one or two extra hits/1 extra model dead.