You have too many variables there to make any conclusion about whether your problems were due to any limitations of MagicJack, or other "issues" with your system.
The fact that others had more success does rather point towards "issues" with your system, though...

As already noted, it does rely upon you having a suitable internet connection;
It also relies on your PC being up-to-spec, and not having other conflicting apps, etc.

By way of comparison, have you tried Skype or similar?
If you get similar issues with other VoIP products that, again, points to limitations in your systems...