Without SSD, those machines really lag if you use a modern OS. And you want a relatively modern OS for compatibility reasons for browsers etc, even for just surfing and watching YouTube and Netflix.
I have a top of the line 2010 27" Core i7 iMac with 12 GB RAM and it's still a lag fest when there is significant hard drive utilization. Just waking from sleep sometimes felt like it was taking an eternity.
That said however, I wiped the entire drive and did a really basic install, with just Sierra 10.12, Office 2011, iWork, iLife and the various browsers and Microsoft Silverlight (for Netflix), and it's much more usable now. Still laggy, but not as bad as before. Seems sad though, since the install is only 20 GB, and it has a 2 TB hard drive.
You could do that for a basic surfer / video watching machine.
For the 2009 24", it would be OK, but I believe they won't run anything past OS X 10.11. That's actually OK though for compatibility for now. Just don't expect it to be as OK a couple of years from now. In contrast, the 2010 will run High Sierra in the fall, and other than the HD lag, it will run it quite well.
Note though if you're brave and/or handy, you can put an SSD in these machines.
