First off, I have tried the two other group deals components currently available for Joomla and they don't even begin to compare to CMGroupBuying. They cost more (one of them costs MUCH more) and they do far less. Additionally, the other two components are full of bugs that the developers either refuse to or do not have the skills to rectify.
With that said, I give CMGroupBuying 5 stars, 2 thumbs up, 5 mics, whatever the highest ranking available on your scale of choice. It is quite feature rich and the developer releases a new version (with new features and fixes for any issues reported by license holders) every month.
Yes, paying for a new license every month can get a bit expensive but if you only need/want the features of the current version then you'll never need an upgrade/update. Of course, you can also pay for a license to a longer license (or even a lifetime license).
As for bugs, I only have found one. I had an issue with CMGroupBuying and Jomsocial but the developer responded to my support ticket within an hour, pointing me to the forums where the fix had already been posted.
I also happened to run into two other issues but both were a result of issues on my end. Still, the developer explained the cause of the issues and provided the solution to each.
I recommend this extension 110% to anyone wishing to run a group deals site. Keep in mind this won't give you a "groupon clone" but for the most part, you should be happy with your purchase.