To add to this, a filtered master socket had have a dramatic effect.
It's as simple as how and where they perform their filtering that allows this.
Lets take my simple setup here at home and why this made a change. A master socket entering and terminated on the floor by my front door. From this a hard wired extension cable running under the floor, up the stairs to a front bedroom. The modem is attached at the master, we have no phone attached to the line, so the upstairs socket is not in use.
Attaching a standard filter to the master socket leaves my internal cabling attached, even without anything plugged into it. This is a nice little antenna for all the RF near it between the front door and it's journey throughout the house.
My SSFP (Filtered master socket), does the exact same job as the plug in filter and at the same point, but it also filters that internal cable that isn't in use. No need to add additional filters, but it also takes care of the RF and other loss this cable introduces (well, not completely, but compensate around 99%).
Had I added a filter upstairs instead, I would have left the cable itself unfiltered.
I used to have one from clatiy.it (modified one) and now use ADSL Nation one, which offers the same modification at point of manufacture. As with any filter they can break. I have a filtered master socket on my desk in work, proving they can break (it introduces a loop fault, which had a knock off to the phone as well).
Would I recomend one? Yes, simple as that.
