Jason Snell of Macworld reviews the iPhone 4S and covers Siri:

It’s a major step forward, though there are gaps. Siri can tell you that you have new email, and you can use it to send emails, but it won’t read your emails to you. (It’ll only read text messages aloud.) And while iOS 5 adds the nifty Notification Center, which gives you granular control over how different apps notify you about what’s going on, there’s no option to read alerts out loud when you’re in hands-free mode. A missed opportunity.

The reviews for the most part seem positive, but there are certainly limitations. This is obviously just the first go at this, but for the most part the pros seem to outweigh the cons. I was hoping for reciting for e-mails, but I guess they can’t all be winners.