I downloaded the desktop app from this site and it shows large flashing ads for Cialis, PC cleaners, etc.. The installer will also try to get you to install a number of questionable third party programs.
There are all sorts of weather apps you can get that don't do this.
Being supported by ads rather than tax dollars like the NWS, you get some really bad ads once in a while. Additionally, it's less reliable and they've bought into the use-Flash-for-everything rut that just opens security and stability gaps.
Love being able to look up the weather anywhere in the USA and have an informative and accurate forecast. Not only does the site provide detailed forecast, but there are also weather related stores and a lot of interesting stories on the site also.