On my Jag, it was worth having and saved me £2K (I still had to pay £2K, but still...)
This E350 I have, was the thick end of £800 for Tier 1 (12 months ago on a 2010 car) with a £250 excess - however I've just had the renewal letter and it's now £549.
As mentioned above, you'd know if you have a lemon, and my excess covered a few sins:
1) Loose turbo hose (whooshing under power, but no loss of power) - a trip to the indie (free)
2) Oil leak x 1 - suspected oil cooler - actually another loose hose (about £50)
3) Oil leak x 2 - again, suspected oil cooler - actually the OTHER end of the same sodding hose (another £50)
4) Engine warning light - MAF sensor - new airbox (MAF contained within) - £400, but I paid the excess on that and let Merc do it
5) Glow plug light - failed relay - £150 + VAT - and I paid direct (had that on my 70K CLS too)
Other than that, it's all wear and tear - my car was 50K miles when I had it, and I've done pads/discs/sensors, engine services and an ATF service since I had it.
I shall probably get the warranty this year, and if it goes down again, maybe the following year - but after that I think it's probably just "whatever comes"

(esp if I find the unicorn Brabus box...)