Nye - one big advantage of the BM panniers is the expandable bit. You can wear them in "city" mode and they don't intrude on your filtering abilities, but if you do need to carry something bigger, then unclip 4 plastic fittings and slide them 3" wider - a brill feature I wsh I had on the "lardy".
The BM panniers are also a really useful shape - I've been camping and touring 2-up with tent, airbeds, cooker, clothes, food - no problems. But as Trevor says, they still don't fit a full-face lid in there!
Another thing, if your commuting to work and you use a laptop, then the BM cases are also a perfect size to fit your laptop case - the lardy-leven-fifty cases are just the wrong shape for just about anything!
You may also want to think about how easily you can remove the cases - the BM ones are easy - but, as Reggie found out on one memorable ride out - if your panniers don't fit through the same gap as everyone else's do, then they all wait laughing while you unhitch your panniers, carry them through the gap, and then put them back on again!!!
Bottom line from me, yes they "dent" easily, but wish I had my 650 BM cases on my lardy-leven-fifty!