Well it tells you some fun stats about your phone, but its not all that helpful really as there isn’t too much you can do with them. It does tell you your IP address though, which is handy occasionally and you can ping and trace sites with it if you really feel need, but it is mainly if you need to see what your server is doing.
If all you really want is the free memory option, then you could try the others, but never even thought about freeing the iPhone memory until I saw the button in iStat, and so I have never tried any of the other apps and you don’t really need to free the memory manually.
iStat is a very nice quality app and judging by their updates to their iStat widgets it will hopefully get some nice updates in the future.
Hope that helps in someway.