I’ve been running Microsoft’s free anti-virus solution for quite some time now. I’ve been a fan of Microsoft’s security attempts beginning with their buyout of Giant Anti-Spyware way back in 2004 (then imaginatively named Microsoft Anti-Spyware). Since then, service-started protection was necessary and thus Windows Defender was created to serve that purpose as well as provide the necessary x64 compatability as well. They essentially rewrote the entire program.
Since then, Microsoft has made the largest leap of all by providing free anti-virus to anyone running a “genuine” version of Windows (i.e., not pirated). For most users, nothing will change since most hardware vendors will not equip their machines with MSE since it will not subsidize the cost of the machines as do Symantec products.