Okay I still stand by my comments below, but I did download it as this app seems to be the only one that can connect to SNMP to get TRUE internet traffic INCLUDING that by other clients on the same LAN.
The UI looks nice and has all the information needed in one place.
The NetUse Monitor Assistant aka SNMP Test Utility was/is really useful in determining the right network interface (which turned out to be difficult, but digging through the router pages revealed the correct WAN pipe.) I was actually baffled by the amount of interfaces our dd-wrt mega router (Linksys WRT600N) shows.

The app crashed once but picked up on the data after relaunching it.

Here are a few suggestions:
Billing date start day is fine, but maybe should be expanded to select the time-zone as the zero-hour usually starts at midnight where the ISP is located.
The MenuBar icon is too wide. Have an option to switch off the Counter.
Actually if the whole BW window could be dropped down from the Menu it would be fantastic, as one would not have to switch Spaces.
Allow for choosing the notification sound from System Sounds.
The app really needs a log to document the usage over several month to a year. Something that can be copied/saved/printed for proof/comparison versus the BW metering crap of the ISP. Having a constant quarrel with them and no proof until now.

Great app! Keep on fine-tuning.

I purchased the product and now have been using it for approximately a week.

There are some missing elements, especially in the interface that seem like unusual oversights. The most notable example is there is no cmd-w shortcut or menu option I could see to close the main window without quitting the application. This may seem minor, but it makes overall use of the product less friendly. The analysis is fairly standard, intuitive use of the features that are there, though the graphing algorithm for handling traffic download when the software isn't running seemed unusual to me. It seemed to presume that the entire time frame was at the rate that was either when the program quit, or the first data read when it restarted. I somehow doubt it was averaging based on time and difference in download volume (though I could be wrong, this is based more on "feel").

I did find myself wishing this could measure more than one interface. That can be useful when trying to track down a traffic hog at home. Other items on the wishlist include having advanced features on an advanced features page of the preferences, instead of having to dig through a plist file, help inside the document (with pointers from the actual application functions so I don't have to guess where they exist on the web page). While these may seem like minor items, the little things often become important. Fortunately, some of them are trivial to fix.

I did have one or two odd hang/crashes, but that could have been my system. I was too busy to investigate further.

Nice and useful app. Working fine here (iMac QCi7 3.4GHz - Lion).

Thanks for this update, Andrei. :)

This looks interesting, but no demo available? I know it's not expensive, so if it didn't do the trick then it's not a whole lot of dough lost, but the opportunity to try it for a few days would be handy.

Looks very promising! Needs a bit more information regarding which interface to choose, and a way to select a different interface without restarting. Perhaps if AirPort Extreme or TimeCapsule interface settings were explained as this software is for Apple computers and sold on the Apple App Store?