I use 1520 Pelicans on two bikes. In the States they're cheap, but I assume you've done your comparison shopping. I mounted them using stainless u-bolts and threaded knobs so that they're relatively easy to take off, but I very seldom bother. There are certainly more elaborate mounting schemes, but I'm into fast-and-dirty.
Pelicans are lockable, waterproof, easy to work with or modify, and indestructible. Their only real disadvantage as topboxes is that full helmets don't fit. This would definitely bother some, but I'm adjusted to the absence of helmet storage and don't even notice.
Oh, and for those who care about the looks of their bike, Pelican's are probably not the most elegant. I've got one on a KLR, where it looks sort've appropriate, and one on a Vstom, where it does not. I don't really care, but others might.
Hope that's helpful.