In terms of speed, Leopard is slower than Tiger because its a 32/64 bit hybrid.
Tiger was 32 bit with some limited 64 bit support, however with Leopard they brought 64 bit up through Cocoa. It is this hybridity that bogs down Leopard. Snow Leopard, kernel and all will be all 64 bit so you should see some nice performance and stability increases.
The GUI, I love it as it is. Looks really professional and really clean, not as Flashy as Vista but far more functional and far more consistent. There were some rumours about a new interface codenamed marble which sounds promising.
Tiger was 32 bit with some limited 64 bit support, however with Leopard they brought 64 bit up through Cocoa. It is this hybridity that bogs down Leopard. Snow Leopard, kernel and all will be all 64 bit so you should see some nice performance and stability increases.
The GUI, I love it as it is. Looks really professional and really clean, not as Flashy as Vista but far more functional and far more consistent. There were some rumours about a new interface codenamed marble which sounds promising.