I'm also very interested in this question. I've been running-commuting a lot. I carry a laptop (an old Dell Latitude), with a pack large and strong enough to carry the laptop plus extras (book/s, toiletries, at least minimal change of clothes) is a nuisance --- feels heavy, big, restrictive, uncomfortable, but I guess this cannot be avoided carrying such a load.)
I've experience with a Jansport Syncline 38, and a Gregory ultralight adventure racing pack.
I've had my eye on the Arcteryx Blade for quite a while -- comes in two sizes, 13 and 21 (liters). The 21 Liter pack may hold suit, shoes, toiletries -- I'm not sure. http://hermitshut.com/arbl13da.htmlhttp://hermitshut.com/arbl21da.htmlhttp://www.mountaingear.com/pages/product/product.asp/imanf/Arcteryx/idesc/Blade+21+Daypack/Store/MG/item/346156/N/4294967261%20651
<b>The Jansport Syncline 38</b>
I think 20 Liters is about the size of my current pack --- a Jansport Syncline 38. My laptop is unpadded; the main pocket is huge --- can hold several textbooks, thermos, laptop, jacket; or a change of clothes & laptop. Additional pockets are good. The GREAT feature of this pack are the cinch-straps on the side --- I can tighten the load down to keep it from shuffling. Still the capacity is so large, I keep the cinch straps completely tightened (which works fine, the load is held tight.)
There's a hydration-bladder pocket, which I like very much; internal pockets; the way all the pockets are arranged helps structure the load so it doesn't all lump together at the bottom. I keep camera and gloves in side-water-bottle pockets, knife attached to one of the shoulder straps (which can also route a water bottle tube), etc.
The straps are very wide and strong --- maybe a good feature for a running pack, as it spreads the pressure over a greater area. And the pack is certainly strong.
Still, I'm hoping for a more comfortable load, so I'm going to get a smaller laptop, and switch to the Arctyeryx Blade. I'll be getting both the Blade 13 and 21, for different loads (plus my wife will use the 13 when she needs it).
<B>Gregory ultralight adventure racing pack</b>
I don't recommend the Gregory ultralight pack. It was far too thin and lightweight for me. Maybe it's superb for hiking and carrying clothes and equipment, but it was far too light and open for a laptop and/or books, and loads could not be structured or cinched down for shiftless carry.
I've always been curious about those "ranpacks" advertised in the back of running magazines --- specifically designed for running, and it looks like they really tighten a load down to compact, efficient space. But I haven't seen any reviews. Yet --- they must sell, because they've been running thos ads for SO MANY years... ?!?!?!