Bought one of these recently from a famous high street camera retailer, which cost less than Amazon here (shhh!) and was shocked at how large it is. Echoing the previous reviewer's thoughts, it adds a fair bit of bulk to the end of the lens and when reversed, it adds a lot of extra girth to the lens so bear this in mind if you have a tightly packed kit bag etc. The inside is velvet lined to maximise light absorption, and clicks into place nicely.
The only downside with this lenshood is that due to the ring-USM design of the 70-300 IS USM lens, be very careful when attaching and detaching; the end of the lens moves when focusing so you want to make sure you've got everything held down tightly to avoid stripping the gears inside the focusing motor.