I used to but the conversion rates just don't make it worth it for me. I think it depends greatly on industry though as a lot of it has to do with fraud.
When I see the made for adsense sites with useless content and adsense ads everywhere, I wonder if any clicks are legit or if it is a bunch of people clicking on each others ads! I could imagine you can burn a ton of cash with no results by turning it on.
When I first started using Adwords I was in the "always" category. Not anymore. I did a test earlier this year. I created a new campaign that only ran in the content network. At the same time I created a clone of that campaign that only ran in the search network. I let both campaigns run for 2 months. Content Network Total Spending - $8,475 and change. Leads Generated - 5 Converted - 1 Search Network Total Spending - $6,112 and change. Leads Generated - 164 Converted - 58 I used the same ad(s) and url...only difference was the network I ran the ads in. Funny thing is that 1 conversion from the content network was profitable enough to make it worthwhile. That's a fluke in my book though - I discontinued the use of the content network for that campaign at the end of the 2 months. The search network campaign has continued to hold pretty much the same lead generation & conversion statistics each month. I'm sure it varies from industry to industry but for 90+% of what I do the content network's ROI is not worth it.
I turn it off, people aren't qualified buyers if they are not typing in the term they are searching for. I don't know a lot about adwords but I know enough to run two campaigns exactly the same, in one you run it as normal but turn off the content, in the other you run content but use the minimum click amount. It's getting two darn expensive to run adwords, if your not in the first 6 positions the conversion rate is next to zero, people who find you in position 12, 20, 30 whatever tend to be tyre kickers. I found this out when my terms dropped out of the top 5 down to those positions. I was doing well then nothing....the problem is the profit I originally made is about gone as Google eats my money now my terms have dropped. Even raising my bid doesn't help, I just can't get my ads back up there
Sometimes = Rarely. Running a campaign spend of $3k-$4k per month I quickly found that my conversion rates from the ad network were terrible... horrible, dreadful... I might as well have just been clicking on the 'donate here' button of every rinky-dink website... Well, maybe not *that* bad, but they were certainly not cost effective when compared to our total average cost-per-conversion (all advertising..radio, print, online ,etc) and were way way off when compared to the ROI for search ads (by far our lowest cost per acquisition short of word-of-mouth). HOWEVER, I accidentally let one of our localized campaigns slip by with network ads turned on and later, when running reports, I caught it... it turned out it did ok. Not as well as search, but cost effective enough on a per conversion basis for me to leave it running on the content network. I even turned content network on for some of my other localized campaigns. But I have to watch them... some do ok enough to keep 'em on the network, others must be pulled back to search only. It's all about cost-per-conversion... pretty easy to figure out if it's worth it or not.