I say go for it, but I'm biased because I absolutely love webOS :-)
But if you take emotion out of the equation, the answer isn't so clear cut. HP has decided to get out of the webOS hardware business, making the Touchpad obsolete, and the future of webOS is still up in the air - they are still updating the software, however. There are a lot of webOS faithfuls that will keep their devices for years to come, but if I were you I would wait until HP announces the future of webOS before making a decision.
As for statistics, WebOSInternals estimates 4.25 million Palm profile activations since the first Pre debuted just under 2.5 years ago. This data does not account for those who activated a profile, and then left the platform. It also does not account for folks who use multiple devices. However, WebOSInternals claims credible statistics despite this because this is pretty much the same metric other platforms use to determine user base.
NPD reported that the HP Touchpad was the number 2 tablet sold during the first ten months of this year, second only to Apple. Only 1.2 million tablets were sold this year that were not from Apple. HP took 17% of the tablet marketshare, which equates to 204,000 Touchpads sold between launch and October. Obviously the firesale had a lot to do with this, and it's unlikely to hold in 2012.
Something else to note is that AdMob was supposed to stop support for webOS on Sept. 30, 2011. I'm not sure if that actually happened.
In general PreCentral and WebOSRoundup are good places to get the latest news on webOS and search for statistics.
Edit: HP has decided to open source webOS. There are still some unknowns, but this is a bit more than we knew last week.