I'm using Leopard on my eMac (1.42 GHz model from 2005 with 2 GB of RAM), and it works pretty well. It is a bit sluggish at times, but it does outperform our Dell Dimenson tower PC in our sunroom (which has a 2.5 GHz Pentium 4 Processor, 1 GB of RAM and Windows XP SP3!) Firefox is becoming a pain on the eMac due to if you try to upgrade Flash on Firefox, it will become unusable and is constantly reminding you that you have an old version of the plugin, but Safari is fine. When watching YouTube videos I mostly set them to 240p and that seems to help.
I can also use Photoshop CS2 and iMovie HD 6 pretty well on the eMac, too! It's a good general use Mac, especially while I wait for my new MacBook to arrive this week...
In fact, the first time I ever tried out Leopard was on a PowerPC Mac, back in late 2008 when we went and upgraded most of the PowerMac G4 QuickSilver 2002 towers at my college's Fine Arts building. Because they used 800MHz processors (they were the 2002 model), we installed Leopard by means of a hack and a FireWire DVD drive. It did seem sluggish at times, but it still worked really well, especially with the RAM upgraded to 1.5 GB on all of them. But then in summer 2009, they replaced the PowerMacs AND the Dell Optiplex towers they used for media editing in the same labs with 24" aluminum iMacs, each with a 1 TB hard drive set up in 500 GB partitions; one with Leopard (complete with iLife '09, Final Cut Studio 2 and Adobe CS4) and the other a Boot Camp partition with Windows XP Professional (this inspired me to try out Boot Camp myself once I got my recently-deceased early 2009 MacBook.)