I bought Forza from recommendations from this forum. I'm a new convert to the 360 after my PS2 finally died on me and also I wanted to play TDU (bought TDU and the 360 when TDU was launched).
As to Forza ... it's a bit like Gran Turismo on the playstation. It's a circuit racer, graphics are a bit old now but what do you expect from an xbox game. In terms of game play it plays just like GT where you do a series of events to win credits and prize cars to the compete in further competitions and buy new cars or upgrade them. The differences to GT are that it has damage on the cars (which does effect the handling) and it has visual customisation for the cars in terms of bodykits, paint and vinyls (a bit like the recent Need For Speed series).
But it was worth the 10pounds it cost me when I bought it.
Comparing it to TDU ... a completely different beast.
TDU is great for just cruising around and having fun ... well that's what you'll find me doing.
Looking forward to Forza 2 tho ...