I am running snow leopard on a 2009 mac pro.
It works perfectly. I have zero complaints or issues. It's no problem at all.
I use chrome and terminal for most of my work, but I do run itunes and vmware fusion and run some apps that give me a tiled interface and focus-follows-mouse.
No problems at all. I couldn't be happier.
I am sure it would all go to hell if I got a current model and tried to run SL on it ...
Snow Leopard is no longer receiving security updates; that'd give me pause in anything but a very firewalled desktop installation.
I upgraded SL -> Mavericks a few months ago when it stopped receiving security updates.
It's slightly worse (I don't like mission control, messages is useless to me) but not as bad as Lion (i.e. you'll just need to disable the stupid scroll behaviour, but performance is good).
On the plus side, you get the option to use new software developed for "10.7+" (heroku's db client, atom editor, swift etc)
Once the developers move, the users pretty much have to. I have an iMac from 2009. I had two OS's on it. Windows 7? Everything still works there. Runs fast, new software is great, etc. OSX 10.6.8? DEAD. It's basically useless. I guess it's nice that apple offers free upgrades, except that they mysteriously make a system that used to be lightning fast extremely slow, even though other OS's seem to run just fine..