Currently there are many different brands offering many different types of pen tablets, there are choices for a beginner(casual user) to a graphic designer.But when I think of pen tablets name of 2 companies pop up in my mind one is wacom and the other is genius.Both the companies have pen tablets with a good response time and mostly have a default value of 1024-level pressure sensitivity for all shapes and thickness control.Wacom pen tablets are on a costlier side but they are well built and sturdy.Genius also has a good quality in its products in general.So if you are a casual user then go for wacom CTK series preferably CTK-470 which is small and easy to fit on a table or in Genius you can purchase EasyPen i405.If you are an Intensive user/Graphic Designer you can go for INTUOS series of pen tablets by wacom and in genius you have G-Pen M712X but here I think the INTUOS series of pen tablets by wacom are better and much more customizable when compared with Genius.
While this might have already answered your question.If you are still in a dilemma let me know the specific purpose for which you will be using the pen tablet and I can suggest you some specific models which might fulfill your needs. keep posting