I too have used MSE on various computers, and my experiences have been very different than yours. MSE regularly catches problems that others miss, but no A/V solution is foolproof. The advantage to MSE is its low boot time and low overhead - it does not slow down your system like many others (cough**AVG**cough) and is regularly chosen as the best free A/V solution (PC Mag, Maximum PC, CNET, etc.). The last review I read put MSE in the top 3 of all A/V programs, and the only free one in those three.
What is does not do is scan incoming emails, so if you are using Outlook/etc., that may be an issue. No anti-virus will protect you from social-engineering, and that's the way most of us end up with viruses - you have to train the user on what to look out for and what not to do - not to rely on the A/V to protect them from everything. Condoms do break, and so do A/V programs!
