Is it still useful to use positioning with grids ?

Grids are very useful for positioning elements around the screen and something like skeleton makes it easy to sort elements for responsive design.

When you say positioning i assume you mean, Absolute/Fixed/Relative etc? to be honest i wouldnt even use that to position elements even without a grid. The main advantages of using Absolute is when you have elements hovering over each other or sticking to position even when you scroll down the page.

Relative is the default which keeps all the elements in the position they are placed, Absolute will allow elements to disrupt that relative flow but still stay in position even when you scroll and fixed elements stay in that position even when you scroll (this can be useful when you have a menu bar that stays in the same place all the way down the page!)

In the middle of typing an answer, Samuel's appeared, and it's pretty much exactly what I was going to say. So I agree!

Also, check out the Build a Simple Website. Under Creating a Website Structure there are a couple videos about grids (quite basic). But pretty helpful for getting started.

I agree too. I'm new to Web Design and while prototyping few websites, I was so happy that I learned about grids from Treehouse. There are many templates and/or frameworks that you can use if you need adaptive/fluid/responsive grids on the web. It sure isn't semantically correct to use grids, but is the fastest way to deliver nice designs. If you're concerned about this, you can use SASS or LESS. For more information about latter try searching for Semantic grid on smashing magazine.com.

I hope I helped a bit.

Grids are useful for set the elements of the page at the right place.Once you make you final vesion of the layout, using positioning is up to you, or the needs of the project...sometimes you won't have to use them...sometimes you'll....but the both together are great tools to construct websites :) I hope it helps