IE 8 has much better W3C standards support, but the biggest change is that IE8 has two main rendering modes - a standards-compliant mode and an "IE7-compatible" mode. The standards mode is turned on by default. There's a Compatibility View button next to the address bar that lets users instantly switch to IE7 mode in case they come across a broken page usually ones designed for IE6.
If you design web pages, use this:
<meta http-equiv="X-UA-Compatible" content="IE=EmulateIE7" />
There are no less than 6 rendering modes in IE8, allowing you to specify IE7 Quirks or Standards modes explicitly, as well as insist on IE8 Standards mode. You can also detect which mode IE8 is in by using JavaScript or conditional comments.
The "
IE7-compatible" mode isn't quite as compatible as it should be so problems will still exist.
Security may be better, but as with all popular browsers, it will not take long to be a target. Even Firefox has its problems as it becomes more popular.
I would recommend upgrading for those that use IE.