I've been using GMail for nearly a year now, and it's the best free email service I've ever used. Now, in response to your questions about it:
Pros: Lots of storage space, a big list of useful features, auto-saves drafts of emails so that you won't have to re-type a long email if your computer suddenly crashes. GMail is also a part of Google's personalized desktop (my browser home page features Google's search engine, links to the 5 most recent messages in my GMail inbox, the quote of the day, the word of the day, and the 3-day weather forcast for my area).
Cons: The rather unique interface might confuse new users or people who don't have much experience with computers/internet.
Reliable: GMail is very rarely down, and when it does go down it is usually fixed quite quickly.
Ad Intrusive: GMail does have ads, but they aren't very intrusive at all. I've been using GMail for so long that I hardly even notice them anymore.