OneCare is solid as a rock. I've never had any problems with it. People who object to OneCare are usually people who object to Microsoft.
Any anti-virus program is effective if you avoid malicious websites and don’t authorize the installation of malicious programs. In actual side by side tests, however, One-Care detects fewer threats than some other programs.
Unless you’ve seen a computer taken over by spyware or Trojans, and unless you know what the user was doing at the time, you really don’t have a feel for what kind of protection is needed.
The current crop of malware masquerades as anti-virus software. The virus installation program pops up looking like a message from your virus scanner. The instinctive reaction is to authorize it to clean your machine.