I haven't fitted one myself, but I've seen them fitted, and it looks pretty straightforward: just make sure you have access to a magneto puller (27mm, if I recall correctly.
Now, as to whether or not they work... Like all things RE, it seems that it depends on the bike. Some people claim amazing results, whereas some people say it made no difference. I actually know of one dealership that refuses to sell them anymore as they have fitted several to customers' bikes over the years, and have not noticed any difference.
Just my tuppence worth....