PSX games look pretty decent on a PSP. I've found ePSXe looking better when processing 3D, but nothing beats the portability of a PSP. You almost never run in speed issues with PSX games. I barely consider it emulation because it runs so well. It's almost as if playing PSX games are a native feature of a PSP. However, you may run into compatibility issues with certain games on certain firmware numbers or what they call "pops." Some games run perfectly on certain firmware versions and an updated firmware doesn't necessarily mean better compatibility. Which is why you get the Popsloader plug-in to load different pops for each game.
I hope that answers your question.