The final version is essentially unchanged since the beta release. In my review of the beta, I concluded that the program is a clear success because of its set-and-forget simplicity and its lightweight operation that makes sure your PC takes no performance hit.
In that review, I noted that the software "shares the same engine and signatures as other Microsoft anti-malware products, including OneCare, the enterprise-focused Forefront and the monthly Microsoft Malicious Software Removal Tool."
OneCare was one of only three tools given the top Advanced+ designation by the AV-Comparatives Web site in May tests.
However, since then, OneCare has not been so successful. In August tests by AV-Comparatives, it didn't do nearly as well, and was only given the "Standard" designation --- below Advanced+ and Advanced, but above the merely Tested designation. It trailed behind 12 of the 16 products tested. It did poorly in that test because it missed a higher percentage of infections.
Those tests were done more than a month ago, on August 10, so Microsoft may have well improved the product since then. And Microsoft has discontinued OneCare, while still developing Microsoft Security Essentials, so it may be that Security Essentials will do far better than OneCare did in the next round of tests.
In addition, when Security Essentials was released as a beta, AV-Test GmbH tested the beta and told Computerworld that "All [viruses] were properly detected and treated by the product. That's good, as several other [antivirus] scanners are still not able to detect and kill all of these critters yet." That's certainly a seal of approval.
In the coming months, more lab results will come in about the effectiveness of the shipping version of Microsoft Security Essentials. But I've learned over time that you can't go by the results of only one test. After all, OneCare was rated a top performer in May by AV-Comparatives, and then in August rated poorly.
I've been using Microsoft Security Essentials since its beta, and have found it effective -- it's caught several nasty pieces of malware. Now that it's shipping, I'll continue using it, and keep checking lab results to see how well it fares.