Had a few sets now, Givi, SW-Motech and BMW Vario's
Vario's were fiddly to put on / off and the variable size a bit of a gimmick, but apart from that they were nice with a good feel, the SW-Motechs on the Tenere have not been heavily used yet, but I do like the looks and top loading, the Plastic Givi's I had on a sportsbike also worked fine but the odd shape made loading a bit awkward and left wasted space. Based on that lot:-
1. I really like the easily detachable SW-Motech Rails, as I only use the panniers a few times a year it is handy these come off in ten seconds.
2. Capacity is a personal thing, but I fount the @70 litres of my BMW / Sportsbike a bit small for two-up touring, the 82L combined (37 / 45) of the SW's is about perfect, of course you always want more, but I think this is a nice balance between enough room and not allowing us to carry too much, and I think are an OK.......
3. Size / Shape - I don't know why so many folk moan about width, I think the Yamaha ones could have been an inch or so wider and it would have had virtually no impact on filtering, yet would have given a very useful boost to the capacity (@15% :question: you do the maths) certainly my SW kit is not all that much wider and is almost 30% larger. The biggest difference to filtering comes down to how close the boxes can be mounted which is where with both BMW and Yamaha OEM stuff wins as it is specifically designed for just one bike and latches on very close to the bike where all third party stuff hangs a few inches out on rails.
For me I go on Holiday where there won't be traffic to filter through, if I wanted to Holiday in London, New York or Paris I would use the Underground / Tube system.
4. I did like the way the BMW boxes could be closed without locking them, I do find it mildly annoying that when out and about for a day I have to unlock my SW Top Box to get into it
5. I also prefer Top Loaders, find them easier to stuff full and all your gear does not fall out if you need to open them when on the road, I also found the BMW side loaders could be hard to close when on the bike as your stuff settles at the bottom and often ended up having to take the box off the bike to get it to close/
6. Real world testing - The kit must work well, with my SW setup the top box sits way too far forward, OK for solo use, but really cramps two people up making the bike very uncomfortable - I would suggest they never even tested the bike two up, I had my Alu-Rack machined out and the locating pins moved, you should not need to do this due to a glaring design flaw on mid range kit (in fact even on cheap kit!) You also often hear of panniers which get in the way of pillions legs or footpegs
6. General Quality - They need to feel good, my SW Stuff is quite nice, and would probably get an 8 / 10 from me, but the locks feel very cheap and nasty and seem to ruin the impression of quality, on a £1500 setup why did they fit such rubbish locks.
7. Ruggedness - I don't really ride off-road, but with these bikes the boxes (and mounts) should be able to survive being shaken about and the odd drop without the kit being wrecked, I know this is tricky as there are so many different types of crash and nothing will survive every scenario, but at the very least they should be easily able to withstand the bike toppling over at low speed, and not to fall off / crack when used on bumpy surfaces for an extensive period.
A rack that fit real close to take advantage of the None dual exhaust 
