Great thread, as I am trying to decide the very same thing right now.
I was looking at the Trax luggage, Pelican/Seahorse style setup, and Givi.
None of these is exactly inexpensive. So I plan to go with Givi's, and I was comparing three.
What I read and saw so far on several forums is while the E21's initially look cheap, performance is excellent, price is good, and they are perfect for commuting. The Givi V35's may look the best, but I don't like the side loading, a little too "wide" for me, and my XL helmet may not fit in them anyways. Plus you have to get the PLXR rails and only the v35 bags will work on them. These are about $330 more than the e21'S. Next is the Givi E41, those look good, but are huge! If you have the money the E21 and E41 will work on the same rack.
For me I think I am going with the Givi E21'S. I think sporttour.com has about the best deal on them.
Their site explains which rack system you need also. I am planning to go without the top rack/box, and you have
to buy the additional kit from what I saw. I think the total was $442.OO for the pair of cases, rack and no top box kit.
Sorry to ramble, just wanted to share my thoughts