I installed ESET Smart Security in my Windows 8 Pro x64 PC and had two problems.
After boot, on login, ESET icon was not there at least the half of times. I followed ESET's instructions to delete a startup value and create another, but the same behavior remained. No other realtime AV's were installed. ESET's protection seemed to work as connection to download Eicar test file was broken but there was no notification about it as it should. Of course this does not happen to most users but it was enough to make me remove it. If you won't experience such problem, it will be a nice solution for your PC. Also, it's not adviced to use two real time security apps simoultaneously in general.
WSA is a respectable product and very very light (maybe the lightest between 3rd party products) but has not the detection rates of ESET. It requires internet connection to work properly as it's cloud based.
WD with WSA is a very good choice. I am currently using this combination-(update: I removed WSA and left only Windows Defender). Webroot says that you can use it's products along with others with no problems. It's not a problem to use one of them alone if you have UAC and smartscreen on plus some third party scanners that you 'll use it once in a time.