I have used Thunderbird exclusively since the pre-1.0 versions. Before the latest version (1.5), I might not have recommended it to people who were really used to Outlook Express, but now it has come a long, long way and IMO, is far better and more reliable than Outlook Express.

It is also safer for your machine in terms of viruses and email attacks.

Here's a few other email programs I've used on Windows. 

Eudora; the client is free to download. I haven't used Eudora in about six years, so I cannot speak to the present state. It used to be a very solid email agent and I can't expect that it has gotten worse. Many large companies use Eudora because it had extra features that integrated with the Eudora email servers. You don't need to buy the server to use the client:

http://www.eudora.com/

They have three modes: buy, suffer some ads, or totally free. The product that you buy has some pretty powerful anti-spam features in it. The free mode is still (IMO) better than Outlook Express. Then again, using a lame duck to send messages is better than Outlook Express.

Mozilla/Netscape: there is the email component in Mozilla or Netscape's browser. ie, Mozilla/Netscape are a browser+email program in one. 

http://www.mozilla.org/products/mozilla1.x/ 

IMO, these are good, but use more memory than Thunderbird. If you've used Mozilla or Netscape's email component in the past, you'll instantly recognize the user interface in Thunderbird. I used to use Mozilla for email and switched to Thunderbird. Haven't used Mozilla since.

The last email client for Windows that I'll mention is Pegasus Mail, aka "Pmail."

http://www.pmail.com/

It's very fast, light and stripped down as well as pretty bullet-proof. It's bullet proof because they don't go in for lots of "let's integrate your email content with the rest of the system" that Outlook Express (and even Mozilla/Thunderbird) tries to do. It does best on plain text email messages and for that, it's rock solid. It lacks a Bayesian spam filtering facility, which I really require in any email client now. Pmail also suffers from a somewhat non-intuitive user interface - you have to hunt to find how/where to make various common features work. Once you learn it, it's no problem, but for beginners, this is really annoying.

FWIW, I find that no program on Windows is "excellent" for email. When I lived/worked/breathed/slept in a Unix environment, the email processing programs available were so much better than what is available on Windows or the Mac that for me, going from Unix to Windows for my email has been like taking a step backwards 10 years.