I tried to use their service to do that recently since I do like the GMail interface, but I couldn't create an account. The domain name had apparently already been used with them by a previous owner. There was no obvious way to fix it other than to kick someone else out of their gmail (as far as I could tell -- I'm not sure what exactly would be reset in the process), and no way to contact customer service unless you already had an account. Since I couldn't safely create an account without talking to customer support, and couldn't talk to customer support unless I created an account I was stuck in a catch-22. By that point, I realized that I don't want to deal with yet another god-damned unsupported Google product and made other arrangements. Hell, I would have been willing to pay $5/mo at the time too -- it'd be a lot easier than what I had to go through to get my mail working. Maybe their support is fantastic once you get an account working; I don't know -- I couldn't get that far to find out.
Yes it is. The SLA is 99.9% uptime until you do something against their terms of service, but even then you'll still have somebody call and complain to.
Also, I'm not positive, but I'm pretty sure it's standard to set it up on your own domain and forward data from any gmail.com addresses you care about.
I don't know if you can port an @gmail.com email address in, but if you have your own domain for $5 / user / month you get phone + email support.