I'm primarily a web developer and designer, so I do most of my work directly in Photoshop/Gimp cutting and clipping and filtering. I eventually stumbled upon the opportunity to purchase a very inexpensive reconditioned Dell Latitude XT, and my experience has been pretty positive. It allows me to much more easily create masks, draw layers, make my selections, etc. The pressure sensitivity is a huge plus, as it frees me up to make minor tweaks without having to constantly re-size my brush or keep my hand on a slider while I mouse-draw.
A few things to be aware of:
1) There are a variety of sizes and styles of tablets, but the industry standard is mostly assumed to belong to Wacom. You can see their products here: http://www.wacom.com/ . As you can see, they provide products for both industry professionals and home/office users at a variety of price points. I would recommend trying out the Bamboo series as a first tablet.
2) If you choose to purchase something other than a Wacom device, be aware that they use a different API to interface with the tablet program and your OS than Wacom does, and they are not completely inter-compatible (especially with Adobe's products). NTrig, the makers of the touch-screen technology for both the XT and the HP TouchSmart have released new multi-touch and pressure-sensitive patches for their devices, which also makes them compatible with the WinTab API, so this is no longer as big of an issue, but it still causes compatibility problems with certain applications.
3) It mostly imitates the feeling of traditional media, but you'll find that it's not a true replacement. I'm terrible with a pen or pencil these days, but my dexterity with the tablet has gotten a lot better; these skills are not completely interchangeable.
Good luck, and enjoy!