It may not be the full monty, but apparently it is still the "definitive automotive experience in gaming".
From IGN UK:
"Deeply flawed as the structure of the game may be, there’s no doubting it still offers the definitive automotive experience in gaming. As a taster for the full course it suggests that, when it arrives, Gran Turismo 5 could well be one of the greatest driving titles ever. But as a standalone title it’s impossible to not question Polyphony Digital’s decision to drip feed its new game and present what at times can be a ragged package. That said, there’s enough content here to keep the petroleum levels in the bloodstream of the most ardent automobile fans peaked, and as such it still manages to nose clear of the rest of the chasing pack."
http://ps3.ign.com/articles/862/862436p3.html
It's a matter of choice, really. You don't HAVE to buy it, you can just wait for the full GT5. Personally, I think it's reasonable to pay $39.99 if it indeed is the "definitive automotive experience in gaming", even if it's clearly not the final product.
Europeans seem to agree, by the way. It's still the number one selling game in Europe.