Like another poster here I made a lot of use of processor upgrades personally in the days of socket A: 1800+, 2500+, 3200+.
For customers I've made a little use of it in the era of socket 939/AM2, going from single to dual core.
AM3 has yielded the most benefit for customers, going from say an Athlon II X2 250 to an X4 640 or X6 960T.
I once tried to make use of it on my wife's Intel rig; she started on a Pentium G620 (Sandy), the board (ASUS P8H67-M PRO) listed Ivy compatibility, I picked a processor on the supported list (i3 3220 or 3225 IIRC), did all the required dances and jigs of double BIOS updates etc, nothing. A year or two later I picked up a 2550K for her on ebay which should do her until the end of that board's days.
So mainly in a business context I would make more use of it if I could, but since I got burned I'm a little more shy of trying it even if official support is listed (ASUS were of no help, I expected this frankly). I'd stick to the same generation of CPU and that's it.
Since embracing Intel for my PC builds since Sandy came along, I've normally recommended i3s until Kaby, I made use of the G4650 once IIRC, now it's either a Pentium or an i5 depending on the customer's budget. I build for the long term generally speaking, the vast majority of my builds have still being going after 8 years, I replaced a 2004 build last year though which I was quite impressed with
As I try to recommend a good long-term processor choice these days I doubt I'll be doing CPU upgrades much in future, though I suppose if Intel i5s drop considerably in price before they reach paperweight status there may be some Pentium/i3 -> i5 upgrades on the cards at some point.