Is it worth changing competitive ad filter often? Do changes apply instantly? Day traffic from US, late night traffic from overseas.. it may make sense to block europe ads during the day. -- Is it possible to filter/unfilter some advertisers based on channel?
I guess it depends on what you mean by "change". I update mine frequently to help kill unwanted ads. Changes take effect after a few hours, 3-5 by my best estimate.
I only have two URLs in my filter: one for an affiliate I sell on my site (and which makes me good money, so I don't want to lose it to some other bozo using Adwords to sell the same affiliate), and the other for that crappy "azhomebiz" (or something like that) which just takes up whole chunks of your inventory for chump change. Other than that, it's all good.