ok, wasnt sure which you meant. i knew you didnt mean methane, which is swap gas haha.
the good-
can run higher boost levels on pump gas
helps keep inside of the motor clean
has some cooling effects
its cheap compared to race gas
the bad-
even tho it allows you to run higher boost than you would on straight pump gas, it still isnt as good.
it is a failure point. if you are running high boost and something fails, there is the potential for kaboom.
sometimes harder to tune air/fuel
hard to get even distribution between the cyl.
there are quite a few nice systems out there. everything from software driven to progressive controllers. some even have some nice "system warning" features where it will back off timing or boost if there is a problem. but it is still a failure point to some degree.
it can be beneficial, and ive seen cars work well with it. but ive also seen problems with them too.
i use a mix of 93/110 in my car. yes, it is more expensive. but i know each cyl is going to be running the same air/fuel, and there isnt a failure point.
hope this helps