An antivirus app will have a major effect on your battery life. A friend with an HTC Desire couldn't understand why he could never get more than 4 or 5 hours out of his battery, even turning most things off, until he remembered installing one of the AV apps to try it out. Removing that brought his battery life back up to what it should be.
When the first Android troojan virus hit back in August last year, similarly to the recent Geinimi trojan, in both cases the affected user had to willingly install an app that was from a third-party source, and go through the permissions screen where the app specifically asks for permissions that it shouldn't really need.
An attentive and careful user can protect themselves from these kind of threats just by taking care to only install apps from trusted sources, and to read which permissions an app asks for as it installs, and stop to think whether it really needs to do things like read your contacts and send SMSs.
The real problem will come when a virus learns to either break out of its app's sandbox and into another, or learns how to give itself root access, or take advantage of the root access that someone else has installed. This is what the iPhone worm of about a year ago did, when it took advantage of the fact that the most popular iPhone jailbreak app left all jailbroken phones with the same root password by default, and most users never bothered changing the password. No Android virus has yet been shown to be able to do that.