We have a pair and use them all the time. At least you know if you can squease it in the bag it'll fit in the pannier - a struggle in itself with the amount my missus takes
I would say they don't make best use of the odd shape of the panniers. We generally have enough room around the top to fit a fleece in.
Also the side pocket is deeper than the recess in the pannier, so if you fill that right up it'll stop the bag going in fully/properly.
Even though we have the bags we still tend to take the panniers into the hotel room. Force of habit I suppose.
I'd say if you can get them cheap (less than 50 quid) they are worth it, otherwise I wouldn't bother.
I also have one of the motobags side bags. Note that the right hand side one won't fit if you have an S as the rear shocker reservoir gets in its way. I also fitted my scott oiler reservoir that side. The side bag is real handy for carrying a small bottle of extra fluid, any extra tools, and a load of tie wraps & tape.
Cheers,
John.