If used with software like Corel Painter or a properly configured Photoshop then you can achieve extremely realistic effects. It's a lot of fun, but if you generally paint impasto and work a lot with textured paper, of course you're going to miss that moving to digital. Some tools like watercolour behave a bit weird in Painter but that's software and has nothing to do with the tablet (although the tablet drivers are excellent and allow you to customise pretty much everything. Even if you have multiple pens it remembers the settings for each one, lets you set sensitivity etc.)
One major issue with working digitally is that one might not appreciate their attachment or comfort brought by working with their hands, touching and feeling the aesthetic properties of their worldly physical materials.. taking this away can create a sort of uncanny feeling that can be off-putting, but is usually offset by the sheer freedom and diversity that a tablet allows (particularly rubbing things out and creating nice smooth lines with effortless media control like stroke width). Tablets I find are best for illustrative and graphical work. They are certainly not a replacement for natural media. Actually one fun thing to try is combining scanned-in natural media and working over that, like papers.
Also you will need to get used to looking at the screen and not your hand on the tablet, which didn't take me too long. Playing games a lot seems to help improve this remote hand-eye co-ordination so it might be easy for you. If you are trained in working on canvas drawing at arm's length in the "art school" way then this will help also because you will be less attached to your hands and more on the image you are creating with your strokes.
But the Wacom tablets are in a class of their own and are a pleasure to use. Certainly worth the money if you are sure you are going to use it. The pressure sensitivity is fantastically responsive and the pens are much better designed than other products (and require no batteries). Using less dynamic media emulations like pencil, pen, dry media etc, and in some cases virtual paint, you wouldn't be able to tell the difference. However, to work with digital paint and big gestural strokes you will want a faster machine so you get the instant result without slowdown (you can sometimes see the strokes lagging behind on screen which is disorientating).
I have an Intuos3 9x12/A4 tablet.
For instance, this is the third thing I ever created with my Wacom in about 30 minutes copying or roughly tracing a photo of my band (I can't remember). The colour is really rough but if you look I think the gestural line strokes are really nice:
This is thanks to the fluidity of the tablet's tracking. My strokes are pretty fast and broad so I am really happy with the tablet's ability to keep up and include small details like the little flicks at the end of strokes where I move to another.
Working on very small areas of fine detail with the lightest of touch is exactly how you would expect, it is very accurate, after a little practice of course. And that's with 2000 levels of pressure sensitivity or thereabouts - the Intuos4 has around 4000.
So I thoroughly recommend the Wacom tablets and wouldn't consider any alternatives they are so good, but as ever they are only as valuable as you use them. Although if you find yourself not getting on with it then they have a very high resale value, at least 60%.