Latest industry stats show that Internet Explorer still holds 76% of the browser market. Yeah Safari and Firefox are making inroads, but not fast enough. Unless you are required to use it, I have no idea why anyone still uses IE.
If you need more info to help you make the decision, the following post should help…
Scott Berkun is a UI designer who worked on IE 1 through 5. Over at his blog he discusses why he switched to Firefox.