I s'pose it must be 10 years ago now, but one of the motoring TV magazines did a comparison between the BMW M5, Holden Monaro and... Something else. This sort of comparison was popular at the time, featuring one or another of the GM group cars, often badged as a Vauxhall, eg VXR8. (Other cars that featured in these test were the Jaguar S-Type, whatever the contemporary hipo Cadillac was, and so on.
The summary, when compared to the GM car? The M5 was typically the better car. But so it should have been given the substantial difference in price. More than one review reported that spending the difference in purchase price on the Vauxhall/Holden would probably see a very different outcome, certainly in performance terms.
For an everyday car - the M5. It is 'the package'.
If you want to tune it to do silly things... Is there a limit to the power that has been made from a GM small block?
These days, a s/h M5 is a bargain. The perceived complexity marks it down, while the national pride and greater simplicity of the Holden means prices are holding up well. A Monaro in NZ costs quite a lot more than a Monaro in the UK at present.