Thanks everyone for their input.
The reason I'm asking is that at LensesForHire.co.uk we're thinking of switching over to using Peli cases rather than foam-lined cardboard boxes for shipping our lenses. Whilst that may seem extravagant and expensive, it can take a lot of effort to make cardboard boxes reuseable after they've been through the post/courier system a couple of times - and time is money, as they say.
The trick is finding the right size. I guess a Peli 1510 would be OK for a 300/2.8 with a pair of Extenders, and maybe a 1450 for a 70-200 or 100-400 or similar. can anyone confirm that?
I guess the main thing I'm concerned about is whether there's enough padding to protect a lens in it. Parcels can be thrown around quite a lot in courier depots, especially if they say "fragile" on them! We currently pack all our lenses in cardboard boxes with at least 5cm of foam all round them, and it looks like there might not be room to do that with some lenses unless we go for stupidly large Peli cases. On the other hand I guess the strength of the Peli case means you don't need so much padding...?