I use the Dexcom SEVEN Plus (http://www.dexcom.com/) and really like it. I've tried a Minimed CGM and found it too bulky for my needs.
There is a sensor that you wear (much like an insulin pump site) with a very small wire that stays under your skin. The wire takes glucose readings from your interstitial fluid, not your blood. With the Dexcom system, you have a seperate "receiver", which has the screen you can see the glucose information on, set alerts, etc.
For me, using a CGM has made a huge difference. I can get alerted when I'm dropping or rising fast, so that I can take action before things get out of hand. It will wake me up if I fall below a certain number (that I program in). It shows me how different foods, excerise, stress, and anything else affect my blood sugar. It's a very, very valuable tool to have.
I should also note that my A1C has lowered significantly in the past year, and I attribute a great deal of that success to CGM usage. My last A1C before going on the CGM was 8.2, and the ones after I started wearing the monitor were 7.1 and then 6.5 most recently.
The CGM is not without it's faults - it's another thing to wear and carry around. It can be frustratingly inaccurate sometimes (although, I'd have to say that about 80 - 90% of the time, the readings are within 10 mg/dL), and some people have issues with the adhesives they use.
If you have any specific questions about it, feel free to ask. I also have a picture of the actual sensor you wear on my Juvenation profile page. You can also browse my blog (http://textingmypancreas.blogspot.com) for more pictures of what the receiver looks like, etc.