Personally, I have had it on a few cars including my previous MX5. Decided against it this time, going to try and stay on top of waxing instead.
I'm still undecided whether the product is worthwhile, but at the usual 300-ish it is a total rip off. My understanding is that it is applied like a wax and then polished off after allowing it to 'cure' onto the paintwork - the product itself costs a fraction of that 300 (I hear of kits regularly going for 10% of that or less) so the dealer must be taking a large wedge of profit, even after paying the labour of the bloke(s) who apply it.
Only you can decide if the benefit outweighs the cost, but remember it won't stop your car getting dirty! If I ever do opt for it again I'll not pay more than about 100-150. Like I said you should be able to find a kit yourself, so if you are up for it you could apply it yourself anyway for a few quid.
Same goes for GAP insurance, the other dealer money maker - shop around because cover is usually available for less elsewhere, e.g. ala.co.uk (I got 4 years new for old cover, gap limit 20K, for a shade over 200 quid) whereas dealer wanted I think 349 for inferior cover. This is the first time I've bothered with it, but i thought that was reasonable value even given the very low probability of a total loss event (i.e. unrecovered theft or write off).