The day after I received this pannier, I went out for a ride and got caught in a heavy shower. On that basis, I can testify that the bag is completely waterproof - and that was without using the bright yellow waterproof cover that comes with the product.
The bag is spacious too. The main compartment is big enough to hold everything I need for a day's cycling: spare sweater, waterproofs, food and drink, first aid kit - plus the aforementioned yellow cover. I carry my tablet computer in the pocket on the top flap, where I can get at it without having to delve into the main compartment. There is a large-ish back-facing pocket which holds a spare inner tube and my small toolkit. The smaller pocket below that would carry, say, a mobile phone. Finally, there is a side pocket where I keep my bike lock.
My only complaint is that fixing the bag to the pannier rack can be tricky. There are three catches: two at the top and one at the back. You have to make sure all three are held open in order for the pannier to slide neatly on or off the rack. I found this awkward at first, although I've just about got the hang of it now.
The bag comes in both a right and a left model. The only difference is the orientation of the two back-facing pockets. You can put either model on either side of the rack, but if you put it on the "wrong" side, the pockets will be facing forward and could interfere with your pedalling.
On balance, I am happy with this pannier, and would recommend it both for day-to-day cycling and for long days in the saddle.