I have a SB6120 on the 30/3 service and have had it for a bit over 2 years now so it has paid for itself.
I am not an online gamer so am not sure what your ping tests mean for games however every test I do shows I get the speed I pay for, not only WOW tests. I watch streaming HD often and all is just fine.
My local speed test is 33ms. I used to be able to do speed tests to all the various WOW nodes but now they all direct me to my closest one. I would like to be able to test others than my local but seems not.
In general my speeds and PINGs are good.
Would be nice if WOW had more MODEM support for ones they did not supply but they seem to not. The MODEM monthly fee for a basic MODEM is sort of a ripoff, IMHO, but at least my SB6120 still works. A MODEM does not really cost all that much these days.
Comcast has a nice list of MODEMs they certify, WOW not. This is the one thing I like about Comcast.