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" />
" 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.