Not worth the time it takes to learn to use them
This is my opinion, and not a humble one, based on more than a decade as a coder. That's not long in the whole of things, but it is a significant portion of the lifetime of the web.
These frameworks, or prototyping helpers, are not really that helpful to anyone who is knowledgeable in html and css. They are actually a bit restrictive, and absolute beasts to debug and maintain. The markup is not in the least semantic, which flies in the face of the semantic web.
Anything good to say? Well, they do allow print graphic designers to prototype without having to become truly competent coders. These grid systems emulate the print industry's paste up board, which is appropriate to print, not so much to web design.
You want a prototyping tool? Get paper and pencil, and sketch the layout. Determine the desired dimensions, without being restricted to a preconceived grid. Then code it.
And to clarify Gary's comment
And to clarify Gary's comment "To maintain" I currently, have taken over a project originally started by another developer where they had taken things through to a finished markup level and styling, the developer had not used a grid layout as such but had gone one step further along a nightmare route and conceived their own grid framework. So the layout main structure is all tokenised with names such as 'three_column_two_row_container second-tier' & 'inside_double_three_column' & 'inside_three_column'. This may look on the face of it ok and I'm sure it made perfect sense to the orig dev but in reality it makes changing any of the site structure hugely awkward as it's very difficult to follow when flowing through the stylesheet and determining what effects your changes may make or where to make them, and make them in a disastrous manner across many disparate page views.
Oh and ought to point out
Oh and ought to point out that we have discussed, many a time in the past, here on the forum, these grid frameworks so you may like to do a search and have a read of some of those where there may have been further opinions and detail on these frameworks.
Tyssen pointed one out that
Tyssen pointed one out that is not too bad. http://cssgrid.net/ It works with media queries and works down to mobile page sizes. The math being done for me is the big draw.
I do have a redesign to complete over the winter so I think I'll give it a whirl.
Basically, what I'm saying is, unlike the other grid systems, I don't hate this one. I can't say I like it yet either though.