With so many options out there finding the right shopping cart is like finding a needle in a haystack.

I'm hoping a few members can chime in with their experiences and maybe narrow down the field for those of us that are looking.

Which open source shopping cart have you found the best experience with considering ease of use, ease to template, ease to mod, number of features, overall processing speed, number of mods available, etc.?
Pros, cons?

Also which carts would you warn against and why?