I had one on both my fazers as I did a lot of miles, including commuting and Europe tours, all year round. It paid for itself after 2 chain and sprockets. However it does need some attention - I found myself riding hairpins in the Pyrenees with oil all over my tyre due partly to the warmer temperature and partly due to my bad setup. They're not a magic bullet. Cost-wise you will never get back the money of the all singing all dancing electronic model - the vacuum model takes maybe 50,000 miles to pay for itself.
I didn't put one on the Street as it's not as easy to hide and I don't do enough miles now to make it worthwhile. I prefer to use gear oil and just be diligent about it. Cleaning the chain is much more of a ball ache if you use "anti fling" chain spray, which converts to a satanic hybrid of glue and grinding paste after a while. Gear oil will fling, but it removes dirt as it does. It's also easier to wipe off the wheel than chain lube.
The only V8 I'm likely to own comes in one litre tetrapaks and tastes like celery.