Everyone I know uses gmail for the same reasons I use gmail: it has a nice itnerface, is easy to use, has plenty of useful additional features, rarely goes down, provides loads of space so I don't need to worry about deleting stuff that I might want later, its spam filter works well and ... its free. Being free is the main thing though, because google could drop one or more of their other features and I'd still use it, but force me to pay and I'd look elsewhere. Why? Because I can.

Not that I'm not willing to pay for a good service - I am (and do), but I need a lot of additional value over what I can get for free. Gmail gives me a lot of value, but I have yet to be convinced that its enough to pay for.