I commute in NYC about 4 miles each way. Lots of traffic, lots of lights. I use an older road bike with drop bars and decent modern rims. When I started this routine, I was carrying my laptop and clothes in a backpack that also tied off around my waist, taking weight off my shoulders. It was great, but more recently I switched to a rack, to which I secure a mid-size bag carrying same load as above with a few other bits. To protect the laptop from road shocks, I line the bottom of the bag with a sheet of packing foam.

All in all, the weight strapped to the rear of the bike definitely compromises handling. It's not nearly as agile, but it's not bad. Most days, I now carry substantial extra weight in the form of a large heavy lock and chain and I just never bother taking it off the rack. Once up to speed, the aerodynamics of the bag secured behind me instead of on my back seems to yield better top speed. I can get lower on the bars, which I like, without the bag getting out into the air stream, nor pulling on my shoulders awkwardly. In the end, it's all personal preference of course.