Choose your tools
Any builder, wether he builds a house, a cabinet or an interface, needs an absolute minimum set of tools which will allow him to prepare his work, get through it, and then check it along some given milestones.
Having spent a fair amount of time in a metal workshop, I have learned to love and identify good tools. To a point that getting in a hardware store feels as exciting as going in a nice restaurant. Good tools for metal work are expensive, so you have to take your time picking the right ones and prioritize your purchases.
The first thing you need is to be able to measure correctly. Without parts having the correct dimensions, you're going nowhere. The logical second is to mark what you've measured, so you can process it. Then you need to position correctly and check the parts of an assembly against a design.
The quality of these tools will determine how long you'll be able to use them. And because with time you'll learn to manipulate them and read them better, it is important to choose durable tools. Your work will be more comfortable, you'll save time and worries, because you have practical and reliable tools.
An expensive scriber - swiss-made-like expensive - will be better because it will remain accurate and will survive you if you take good care of it. Your ruler should be well printed and/or carved so it stays legible for a long time, even in low light. Your square should be the toughest you can get because if its shape changes only slightly, it's no more a square.
Your tools define what you can do, the quality of your production, and the time you'll spend working.
This detour to a hardware workshop is the closest experience I would compare the use of xScope to. Building an interface is not unlike building with metal or wood or plastic, in that it requires you to measure, mark and check your work. xScope does that on the screen, providing you with lots of measurement options and saving you time. If you want to design webpages in the browser, it's a requisite.
Anybody doing interface design on a mac has to have it. I would recommend to anyone doing interface design on a PC or on Linux to get a mac just to have this tool at hand and eyes, as it seems no decent equivalent exist on those platforms.
Also, it is a great way to observe and learn from the graphic work of others, just as reading source can learn you lots about markup and css methods. If you like Dribbble, you'll love Dribbble + xScope.
This is not an advertisement, I'm not getting any money back if you go buy it from here. I just thought this little gem doesn't get as much praise as it should. Thank you Iconfactory & Artis Software, you make our days better.