This chart comes courtesy of this post on Engadget and is originally from Jupiter Research.
As a recent convert to Tablet PC, I am extremely happy with the operating system and the digital ink functionality. It is surprising to me that there is still a slightly higher cost for Table PC functionality in the marketplace and that more laptops do not come with Tablet PC functionality built in; I would imagine that in the next few years, Tablet PC functionality will probably be a standard feature of middle- to high-end laptops and that only bargain laptops will not include it.
An interesting data point above is the 25% that are not knowledgeable of the Tablet PC architecture or that it even exists. Microsoft has certainly put less marketing horsepower behind the Tablet PC OS than it has many of its other products, so that may be a contributing factor to a lower adoption rate.
I will simply say this: The Tablet PC that I am using lives up to all of my expectations. My only gripe thus far is the lack of battery life; I get about 3 hours with the brightness on the screen cranked up all the way when I use it outdoors. It took almost no time at all for me to teach myself how to use the Tablet PC functionality and I have had no problems at all with the software recognizing my handwriting. Maybe it's just everyone else's loss.