For the past week I've been using the latest browsers from Microsoft (Internet Explorer 9) and Mozilla (Firefox 5). They both installed easily and rendered web pages without much new surprise. On one of my client sites all of the text disappeared in IE 9, so a quick Google search uncovered that using Cufon fonts with IE 9 required an upgraded Javscript file to be compliant.
If you love Microsoft then go ahead and upgrade to IE 9. Microsoft is the safe choice with some 54% of all browsers in use today, per NETMARKETSHARE. Firefox works well and is gaining market share against IE.
As a web developer I only use IE as a final browser check for compatibility reasons, and don't develop with it. My daily favorite browser is Google Chrome because it consumes less memory, renders fast, and has a productive Inspect Element capability I use for debugging CSS.