I used MM CGMS for about 2.5 years, and then Dexcom 7+ and now G4 for the past 2 years.
I agree with the others, Dexcom is much better, and worth carrying 2 devices.
One important benefit for me is the ability to put Dexcom receiver under my pillow, and hear it much better. I used to miss MM CGMS alarms at night. The G4 has even louder alarms (using Attentive profile), and haven't missed one yet.
The MM CGMS was 'high maintenance' - took longer to insert and tape up to be secure, had to recharge battery on transmitter, had to be especially careful of when to calibrate, often did the insert at night, then start in the morning to allow it to 'marinate', new sensor every 5-6 days, etc.
I find Dexcom much more accurate, and it responds better to a calibration when it is off, and 'corrects' itself. I just insert it, start it and forget it !
I have been able to get 10-21 days for most Dexcom sensors, with only the 2 hour startup time w/o readings, each 7 days. I calibrate 2-3/day, when it's convenient for me.
Very low maintenance.
I was never impressed with Carelink, and like the Dexcom software better. Although it is a disadvantage to not see the insulin and BG trends on the same timeline, it wasn't too important to me (my doctor missed it more). Since I often do a calibration at my pre-meal BG check, I can still have a pretty good idea looking at Dexcom charts where the food/bolus is, and review corresponding BG trends.
There are some things I missed that are on MM CGMS, and not on Dexcom. MM was nice to be able to scroll to see the specific readings at each 5 min increment. On Dexcom, you only see the number for current, and prior readings just see the dots on the screen. So sometimes it was hard to see if you might be slightly trending up or down, with a flat arrow. MM CGMS(Revel 523) had more 'stats on board', without downloading the pump, for example average Sensor BG by day, high/low, etc. Also had predictive alarms and more options for some of the alarms and time periods for different high/low numbers.