Disclaimer: I live in Canada, specifically, in the Province of Ontario (we get some coverage for infusion sets through our Provincial healthcare plan. We have to pay out of pocket first, and then get three cheques of $600 distributed to us throughout a fiscal year to cover reservoirs and infusion set costs).
I've been using the 7 day extended set for about six weeks now. I have a feeling that the site is losing absorption capacity or insulin sensitivity at the end of day 5. Beginning overnight on the sixth day, I've started noticing that my blood sugars suddenly spike. I will try to verify that this is a problem with the set and not just natural hormonal fluctuations over the next couple of set changes.
If the sets only last for 5 days, I can't say that these sets will be worth it financially. Right now you only get 3 sets in a box from Medtronic Canada, which costs $192 in Canada. If you were to get 7 days, that's only 21 days worth of sets. A box of the usual three day infusion sets gives you a maximum of 30 days for about $235 (mio advanced).
If the 7 day set only lasts 5 days (and I tend to be able to wear a normal set for 4 days, so some people might not even get 5 days out of the extended set), then you're paying $192 for 15 days max. So if you have to double that to get 30 days worth of supplies, which is what I could get with the mio advance, then you're now paying $384, which is $150 more for a 30 day supply of infusion sets.
The cost effectiveness of the 7 day reservoirs will be dependent upon how many days your insulin absorbs effectively (of course if you also have a medtronic sensor, your pump might automatically adjust for this discrepancy, if the sensor works properly for you; but this could lead to problems down the road because you're infusing far more insulin than you really should be in order to cover the lack of absorption). Now if you have insurance that covers all of your supply costs, then it's not really an issue. You could keep the site in for four, five or six days and not worry about the extra cost. I suppose if you can only get 1 or two days out of a normal infusion set site, then the cost might also equal out. So I've got two more sets to try. I think I'll go back to the three day sets though, if the 7 day sets only last for 5 days, as "5 day sets" would be double the cost of 3 day sets. My two cents.