I am here to put a spanner in the works
What Paul says is right, they are useable, and a great way to get into the hobby. But, if you enjoy the hobby and go a little further, they will become 'tired' very soon.
The HPI I dont know a lot about, but I would be cautious of spares availability. I know speed hobbies on the Hatta road carries them as kits, not sure of the spares though....
The Tamiya XB'x are the same as the kit ones, except they are already built and painted and half the fun and LEARNING about the cars, is in the building. Also, no creativity required to paint.... a lot of people here drive the Tamiya cars, so swapping and acquiring of spares will undoubtedly be easier.
Be aware though that both come with pretty low spec radio gear, with limited functionality and no model memory, which means you will need to re set everything if you want to get more than one car on only one radio....
My advice to you, if you have a budget for either one of those cars, buy a second hand one from the For Sale section here (most guys would have made some decent hop-ups on them already) and use the most of your cash to invest in a decent radio/Tx. If you get tired/bored of the car, just sell it and remove all the radio gear and put it in a new/better car....