540EZ

will this flash work on my XSi? also, any thoughts about it?

This flash does not have ETTL and the High Speed Sync; so it will be more difficult to use on your camera than a newer flash. I have never used it, nor read the manual, so I am not sure how you will set your exposure (i.e., I hope you won't have to keep a table with ISO, f/stop, and distance).

The ETTL flashes are much easier to use -- basically just like your pop-up.

It will fire, but that is all it will do for you. You'll need to use a meter and set the flash power manually if you're going to use it as your primary flash.

You can fire it from your camera but you will have to determine your exposure and do a little chimping to get your exposure right. It would be OK in a static environment but I wouldn't want to take it anywhere that might have different lighting.

I have had several and still have one. They work great for off camera strobist stuff. Manual only - no auto modes.
Example: Flash mounted on a balcony in a gym triggered remotely using the Skyports and powered by a Quantum Battery 1 battery.

For Canon flash units, unless you want to only use the flash manually, I think you have to have an EX series.