Originally Posted by WhyFi

I think that a lot of people recommend 105 because, more or less, it functions the same as their higher-end offerings (Ultegra, Dura-Ace) and many of the parts are interchangeable among the three groups. This gives you more options when you want/need to replace/upgrade - it can often be done piecemeal as opposed to necessitating the replacement of multiple parts simultaneously.