I have a late-2009 iMac which seems to have held up pretty well, especially since upgrading the RAM to 12GB.
I only fairly recently updated the OS to Yosemite after running Lion for so many years.
The OS update my have made my Mac a little slower, but if so then not by a great deal. But I could probably do with updating again — to Sierra — as certain programs require a more recent OS than what I have.
I am a bit concerned that running Sierra will put too much of a strain on my machine... or are the requirements not a whole lot different to any other versions of OS X over the last 5 years or so?