I have one of those exact extenders which has an antenna on the roof and seems to work fine, for the most part, for phone calls. But like a Windows machine, it must be "rebooted" one or more times per day to keep it functional especially when using 3G Data or Text Messaging. My problem with the extender was in the form of data. Text messages wouldn't go out or wouldn't come in and would be significantly delayed. It was a very frustrating and why I went with the Microcell. The Microcell is not perfect by any stretch of the imagination and voice quality is limited if you've got some transfers maxing out your internet connection, but it is not a daily ongoing nuisance as was the extender.
Monday, the Internet connection at my office is getting a significant boost in bandwidth and I bet that any "downside" to the Microcell on voice quality will be a thing of the past. Ironically, I'm getting rid of AT&T DSL and going with Cable RoadRunner. Granted, I would have done that a long time ago, but I rent space from a guy that shares the office and it's pretty much his call in that department.
At home, I now have 30 down and 5 up, which kinda makes me want to work at home again and there I don't have coverage issues.