There seem to be a lot of "Gaming" mousepads on the market - some very expensive. Do these make any real impact with today's optical mice over an old mousepad or desk?
It depends on the mouse.
One of my favorite mice is a Laser mouse made by Razer, but the mouse really sucks unless you've got a good mousing surface to go with it. I personally use good mousing surfaces for all of my computers (even when I'm just at work programming, without a gaming mouse), because I like the extra precision.
Professional gamers will bring their own mice as well as mice pads to events. The reality is no surface is frictionless, nor does any surface have consistent friction throughout it. As a result there are parts of the surface in question which the mouse will move on at different speeds.
A good mouse pad will have even friction throughout it. How much friction you want on your mouse pad is usually a matter of personal preference (like tuning your mouse response settings). I usually advocate low friction mouse pads for laser mice, but I know a number of competitive gamers who prefer high friction mouse pads for higher precision (as the mass of a mouse is usually so small that the force difference between high acceleration and low acceleration is small).
I have personally used a gaming mouse pad since I started competing at Counter Strike. I quite enjoy it, but I recommend you try before you buy. Go to a store that has games on computers and play them for 10 minute stretches.
Its also worth noting that a high quality mousepad may require a different cleaning regimen than you are use to. Make sure you find this out first.
Some mousepads offer support besides the actual pad for the mouse, such as a wrist support. These can help whether or not your mouse is optical, if it's comfortable for you, a fact which has actually helped a number of gamers I've known.
In my opinion, the biggest advantage of a gaming mousepad is size. I use to run my mouse off the edge of the pad all the time until I got the giant mousepad by Razor. I also think the small pads with different "surfaces" are mostly just hype.
I prefer my desk as mousepad, I do ALL the view just by my mouse so it often happens that I reach the end of a mousepad.
Advantage of desk: nearly endless room to play, no annoying rearranging of mouse and mousepad.
Disadvantage: if you are eating on your desk you will often have to clean it, otherwise you will find sticky stuff on your desk's surface which obviously get in the way of the mouse.
Different mice may track better on specific surfaces, but with most high-quality gaming mice operating at non-extreme DPI settings, you should be able to use any non-exotic (e.g. not glass or polished) surface. However, more important are the physical characteristics of your mousing surface. The tactile "feel" you get when you move your mouse on a surface can have a significant impact on your aiming accuracy and speed.
There's a lot of different gaming mousepads out there. Some are designed to be silky smooth, while others have a rougher feel. A smooth surface allows fast motion, while a rougher surface allows higher precision. Most manufacturers of gaming peripherals produce a variety of mousepads. While this is ultimately a matter of personal preference, it is important that you select a mousepad that is comfortable and lets you aim quickly and precisely.