Im not sure why microsoft offers a free update though... Maybe theyre hoping to get people use windows store, thus offering it free to windows 7 users, but also windows 8 because only offering it to 7 wouldnt really work would it.
The ongoing theory (I am not a MS employee) is that this is the last major version of Windows and they want to get people onto it as quickly and painlessly as possible so that they only have one platform to keep updating over the years rather than several. Especially since moving to this OS also (mostly) dumps IE for a new browser that doesn't have to deal with all that legacy junk.
I don't really see any reason NOT to update to 10 unless you're running some ancient software that didn't follow MS's guidelines. (I.e. most "enterprise" software) All reports I've seen seem to indicate that everything works just fine in the new OS.
Of course, it's perfectly reasonable to wait a month or four so they can shake more of the bugs out ;)