If like me you have an X300R you'll know all about removing the chute and poking a stick into the deck to clear damp grass.
I bought this as a bit of a punt but I have been very impressed with it. I am able to cut much longer and wetter grass than with the grass box on. I previously used to remove the box and let the cuttings fly out the back which makes a real mess especially if the back of the mower is facing somewhere you don't want covered in grass clippings.
The only downsides are it is quite expensive for what it is (I was tempted to try to make something myself) and it does leave the grass cuttings a bit clumpy if the grass is particularly long but they dry out much quicker and you can go over it again with the grass box on and collect them if you're bothered.
Very easy to store and fit and I've owned it for a couple of seasons now and use it if I've left the grass too long between cuts - I work away from home so it's regularly left a month without a cut and it never seems to dry out completely when it's that long.
If you can justify the price, buy it. It's been worth it to me for the time I've saved with my face up the rear end of my John Deere poking around with a stick.