In a nutshell, it's a replacement for your mobile voicemail which automatically sends a copy of all voicemail messages to an email account of your choice.
Additionally, your phone will receive SMS notifications whenever a message is received, including caller ID info (name & number), message length and caller location.
Their's also a desktop PC software and a few widgets. The desktop software allows you to do some pretty nifty things, such as taking a call while a caller is leaving a message, etc.
As any regular voicemail system there are three options for greetings: standard, spoken name and custom. All of the greetings mention that you are using Callwave, which is fine; after all the service is free. Although I think some people might be under the impression that you refused their call when they hear the "Callwave" name.
If someone calls from another cellphone they'll also be given the opportunity to receive a receipt notification (SMS) upon leaving a message.
On the minus side, you'll lose the voicemail waiting notification icon on your phone, and incur additional airtime (and SMS) charges since Callwave uses call forwarding.
The desktop software windows cannot be moved to any other screen but the main screen, which a bit annoying, but not uncommon. It's pretty amazing the number of software that simply won't deal with multiple monitors.
I also wish the standard greeting would include the phone number called, like Cingular does.