The product I'm discussing is Gaugemaster's pre-ballasted foam underlay part number GM200:
http://tinyurl.com/6btzpd
For those who find the thought of going over every inch of track with a small teaspoon and a tub of ballast scary, this product certainly sounds welcome. It comes as a 5 metre roll of foam which is pre-ballasted to fit 00 gauge track (an N gauge equivalent is also available). The idea is that you fit this as you lay the track, and when you've finished you've got a complete ballasted layout with none of the tedious work involved with hand ballasting.
So, does it work? Sort of. On the plus side, it does make getting a basic ballasted finish a lot quicker and easier. It's easy to stretch to fit track and while you have to cut it up and glue it in place to fit points and crossings, this is still rather quicker than doing it by hand.
On the minus side, I thought the height it raised the track to was a touch excessive - the photo below demonstrates what I mean:
This might be acceptable if you're representing a modern high-speed line, but for smaller layouts and goods yards it's probably too much. You can buy separate bags of ballast to match the ballast they've used on the underlay and use this to reduce the apparent height by making the shoulder a little less steep, but if you're going to do that over your whole layout much of the benefit of the product is lost.
I also found that the nature of cutting up and gluing to fit to points tended to result in a finish where it was no longer possible to get the point to lie perfectly flat on the baseboard - at least, not without lots of fiddling with cardboard shims. Those who have spent a lot of time on points will know that this is a very efficient way to completely ruin the running of locomotives over points. The upshot is that any time you save on hand-ballasting may well be lost on getting points fixed so trains run reliably over them.
It's expensive at around £16-17 for a 5 metre roll, and to my mind it replaces one form of tedium with another - the problem of ballasting track is replaced with the problem of getting trains running reliably over it. This probably isn't a huge problem if you've got a lot of long straights with relatively few points, but if you've got a lot of complicated pointwork it may not worth the trouble.
I think it works out quicker than ballasting the old-fashioned way - particularly if you're as cack handed as I am - but it's not a panacea.