I've been using it for donkeys years with excellent results. I used to do literally what they do on the shows (i've seen it demo'ed elsewhere other than Ideal World and have met the folks that run GL UK at that event) and use it on a dirty car, but rapidly realised there's a better way
I now use it as the "polish" after the wash. Wash is always with Dodo Juice Sour Power which leaves excellent results anyway. Then I give it a quick wipe down with microfiber cloth to dry it down reasonably well, and then use the Showroom Shine to go over it to give it a final coat. ECP's comments about it separating out easily - yes it does - about 2-3 hours after you put the bottle down, but it's supposed to, and it combines back easily and well with a light shake of the bottle.
You can use it on all surfaces - the results are excellent to be honest, but I don't use it on the glass anymore. Glass gets just dried down then another GL product gets used - their excellent military grade water repellent which works a treat. I've barely had to use any de-icer this winter as it's just kept it all on the surface and easily removed.
Also use the showroom shine on the wheels and then treat with another product from them the wheel protector. Seems to make it a lot easier to get the brake dust a grime off next time round.
I'm a big fan, but would say they have lots of sales - bank holiday, easter, fathers day etc - if you look at the GL website then it's often a LOT cheaper than the TV channel shows and often get more for your money. Registering for their club gives you 10% off too - just means they will e-mail you the offers.
BTW - their bug remover spray is brilliant - makes summer cleans effortless. Quick spray on, make a cuppa, come back and they just wipe off with a cloth
HTH.
D.