We hired a VT Exec in Adelaide and drove it across to Melbourne and right back up the centre to home, about 1800K all up from memory. It had amazing eceonmy for such a car, we were getting over 40MPG (assuming the speedo was right) out of it,but the suspension was quite scary, over 140 it would float, in the passenger side you could feel it and it was a relatively new car, 15000Km or so.
What I don't like about the Buick V6 is that is has no torque, I think its ~300Nm. Sure the power is good but the pulling nature of it, pah. The I6 from the falcon is good, with the new ones up to 380Nm and even the old XF I had had about 290Nm at about 2200, great tailout fun. The V6 to me just seems to lack grunt and its very hoary when you give it some, the old Holden Iron Block V8 however, nice design and real grunty. Esp the VN Group A, 220Kw from a 5.0L is bloody good work the only LS1 I've been in (VTII Calais) had alot of go, but only over about 2500.