I would stay away from the hdd fan control, mainly because it isn't a long term fix. Although it is an easy solution now, all it would take is a software upgrade from apple to cause it to be useless... a patch could then be made by hdd fan control, but there may be a time delay, in which you would be stuck with the imac fans spinning at nearly 6000rpm instead of the idle speed of 1000rpm... its loud to say the least.
If you want a hardware fix, i've come up with a solution. I'll expand on it a little without going into too much technical detail. (sorry my reply is a bit late, i only just came across this message online)
For starters, a standard SATA power cable uses only 4 cables, despite having a 15pin connector. This is because the pins are broken down in sets of 3, and each cable type can be combined after it leaves the connector. (the main reason for this is because the current on each individual pin cannot move fast enough on a single pin to power the 3.5" drives, so they have to have 3x 12v, 3x 3.3v, 3x 5v then 2 sets of 3x coms, which are then combined onto 1 cable... so the 4 cables used are 12v, 3.3v, 5v and coms)
Without going into too much technical detail, two coms tracts send the data from the internal SMART disk of the hard drive to the main logic board. The difference between Apples drives and an industry standard drive is that the firmware revision differs... The logic boards in the 2011 iMacs can only communicate with certain revisions of this firmware, and after testing nearly 100 different drives, i have been unsuccessful in finding a revision that works with it without changing the hardware or running the software. (which isn't free by the way, its a £20 download, but you can download a sample of the app to test it beforehand)
With the 2009 and 2010 iMacs that use the internal SMART disk to convey the hard drives temperature, i have found certain industry standard firmware revisions do work with it, but each manufacturer uses a different connection type, as they dont use the SATA power cable to send the data, but instead use a separate 2 pin connector cable that goes to the logic board directly. As there is no central source of firmware revisions that will work with it, maintaining stock of the drives that do work has proven to be a pain, as testing a drive from each stock with a 2009 or 2010 machine is not the easiest option. (suppliers are not able to specify firmware revisions when shipping drives in my experience, so restocking a specific revision that previously worked is not possible)
Needless to say, i found a sensor that operates at the same frequency as the internal SMART disk for the 09/10 imac, and its part number is 593-1029 (you can look it up on www.thebookyard.com) its listed as the LCD cable for the same range, but it is compatible with the hard drive port, as i have done rigorous testing on this. all you need to do is replace the current sensor cable, then adhere the sensor to the drive (then it will act as a surface temperature sensor instead of an ambient temperature sensor)
As for the 2011 iMac problem, the easiest solution was to make a cable myself. So, instead of explaining the whole process in all its technical glory, here is the link to the part on the site. http://www.thebookyard.com/product.php?products_id=10085
As for an easier to fit solution, i am currently working on an adapter solution that will not require loosening the screws on the logic board to fit a replacement cable. (As the SATA ports are on the underside of the logic board by the graphics card port)
I have also written a small blog about it here if you want a bit more detail, otherwise i can explain the process in more detail if you'd prefer.
http://thebookyard.wordpress.com/2013/01/24/the-aluminium-imac-ranges-and-their-hard-drives-2011-hd-problem-resolved/
But all is not lost, as there is always a solution! Even if it takes a bit of creative finagling to do so.
Best Answer
As an IT-service company we use several OSes. We use Freebsd on our internet servers, and Linux, Windows and OS X on desktop clients. I'm personally an OS X user since 10.1(Puma) and have never used Mac OS 9 (I have a strong unix background).
OS X's stability and upgrading smoothness is unmatched by either Windows or Linux! Only Freebsd is comparable.
We usually upgrade live servers with freebsd and clients with OS X immediately after release, and have never had any problems. (I mean any.) This is probably because we don't use any "special" applications, just common Apple products like iWork, iLife, and Omnigroup products, etc.
OS X (the system) runs immediately without any flaws. There can be some problems (as @boehj told above) with third party applications, but 95% of the applications usually work without problems, because developers build them in advance. You can already see applications that are Lion-ready.
So, if you are not a hardcore OS X user, ofc, you can wait a while "to make sure", but from my experience: it is enough check how your application run under Lion (by visitng the developer site) and if they are ok, simply upgrade & go. :) Forget the "windows service-pack" syndrome :).
As Kerri said above - ofc, backups are good. Not just when upgrading, but "just because". In 10 years I have never needed to restore anything because of an upgrade. Ofc, we restored when we moved from PPC->Intel and when switching to new Mac models, but never "just because the upgrade went wrong". I'm pretty sure that apple decided selling Lion ONLY as a download via the AppStore (without media) that the upgrade process would be well tested.