From the features list:
Replace SMS with unlimited free chat.
Connect with multiple IM accounts and have them in one list (Facebook, Yahoo Messenger, MSN/Live Messenger, Gtalk).
Click and share your favorite pictures with friends & family.
...keeps your conversation handy for later use.
Use our popular chat buddies for Twitter, Cricket, Astrology etc.
If you already use Nimbuzz on another platform, logging on with your Nimbuzz account will set up the other accounts on your Windows Phone handset, other wise it's a relatively painless process to register an account and log on to your other networks from within the app.
Nimbuzz's main flaw is not the fault of the application, but of Windows Phone's architecture. Without the ability to run in the background, you will need to have Nimbuzz in the foreground and running to pick up your messages and keep chatting. This is an issue for all IM clients on the platform, so I'd give Nimbuzz a pass on this one, especially as the application is free to download.
What Nimbuzz does offer is a gateway to chat clients that are not naively supported by Windows Phone. Depending on how you socialise, that could make Nimbuzz an important application for you.
Pick up your copy through the Windows Marketplace or the AAWP App Directory.