i was using mse up until couple months ago and i found it has got very update hungry and intrusive, i started using the new avg which is a big improvement over avg 9.0, not download hungry and hardly any of the old annoying pop ups.
I still rate MSE highly. I'm surprised to see jamesd1981 refer to it as update hungry and intrusive. The daily auto update takes around 30 seconds to install... the only clue is a little flurry of HDD activity and the icon changing to the downloading symbol. On my settings a quick daily scan follows immediately, this time with a small pop up at bottom right of screen which collapes after 5 seconds or so. The scan takes 6 minutes with little impact on performance. That is all I ever see of it.
The best points for me are that it impacts little on performance... on a 4 year old laptop (AMD Turion 64mobile) running Vista that's important. The other major plus is guaranteed compatability with Windows... and many threads relate to various issues with other programs.