The changes to the detection and removal engine will be noticeable to long-time Ad-Aware users, with Lavasoft touting faster scan times and better rootkit detection.
CNET/Download.com

Ad-Aware 2008 looks a lot like the 2007 version of the program, but with a freshened-up interface and somewhat more intuitive controls in a few places. Most of the changes, however, are found under the hood, and they are significant.
Yahoo! Tech

It was perhaps the first great advertising tracker purging system, and may have single-handedly redefined the public's notion of malware. Now, Ad-Aware is growing up, becoming more of a fully-fledged anti-virus product.
Beta News - USA

In Ad-Aware 2008 Pro Ad-Watch is much powerful and works like a charm.
Help Net Security