I know this is an old question but I'm still confusing which css framework should I use between Bootstrap and 960gs maybe or not.
I think the old grids have much had their day now and people are looking at things like bootstrap or foundation instead.
As others have mentioned its no good using these unless you have a good grasp of html and css first otherwise you will never be able to dig yourself out of the hole that you may find yourself in. Bootstrap isn't as bad as people make out but a lot of the time it is unnecessary for a small site and is more use in larger projects where a team can work on a design using consistent methods.
In some ways it does make life easier if you stick to the grid and use all the defaults and especially the built in library of jquery functions for the most common tasks (drop downs, modals, tooltips, hide and show etc). However if your design doesn't fit the grid you may end up using custom html and not using any of the grid classes or structure which may be a waste of resources.
There have been a few similar questions over the last couple of months so may be worth doing a forum search for other answers.
I thought you were useless