I used to meditate regularly until I decided to take a break a couple of months ago.
On the positive side it has helped me a lot - lowered stress, better sleep, improved communication, creativity and a lot of the things others have already covered. I want to highlight a few things on the not so positive side.
1) lowered drive at work. I had to take more conscious effort to get myself to take action on things when I previously reacted quickly as I was more "on the edge". 2) feeling very emotional / sensitive. I found myself crying to myself a lot more and generally being affected by things around me. 3) Ignoring certain things that needed attention. I think at some point it started to become a crutch. Some relationships needed my attention / action in my personal life and I found myself doing nothing about it because I think in some way I started using meditation almost as some place I could escape into and feel good internally when externally things weren't great.
In spite of all the above, I want to resume my practice soon as I feel that the fact that I can see the issues give me the ability to address them. I highly recommend the site http://lorinroche.com