Type in your search terms and Google will pull cases containing your terms. The cites included in the opinions have hyperlinks to the cases cited. You can also see how and where your case has been cited.
As an example: type in "retrograde extrapolation texas dwi" and the first case on the list is Bagheri v. State. You can read the court's opinion and check out the cited cases in that opinion. Click on "How Cited" and you can review how the case has been cited and where it has been cited. To find out whether the case is still good law you can find the most recent cites and work your way back to the original opinion.
While the system is still in beta, it is a vast improvement over conducting online legal research piecemeal. By bringing these texts and cases together, Google has made it easier to conduct low cost (i.e. free) legal research.