For all of 2015, with my wife on mat leave, I was able to use my bike as my sole method of commuting to and from work. Honestly, in terms of commuting, it was the best year of my life. I enjoyed every single day of it.
My commute is 20 km each way, which takes me between 45 and 55 minutes, depending on my energy level. As such, my ride is substantial enough that I need to change into my work clothes when I get there. Luckily, my office building has exceptional facilities available: showers, change rooms, secure bike storage, etc.
One of the challenges was getting my work clothes (dress pants and dress shirt) to the office without wrinkling them up in a backpack. Also, backpacks are sweaty and uncomfortable, especially on hot summer days.
The Classic 2.0 Garment Pannier from Two Wheel Gear. I can't remember where I had first heard about Two Wheel Gear, but early in 2015 I started seeing the name appearing in articles more and more. Two Wheel Gear is a great local company with a great concept: a suit bag that's made to fit on your bike. As for the company itself, "the focus is to create the very best bags in the world for professional business commuters, to keep innovating with new commuting bags and to make it extremely easy to bike to work."
On the outside, it looks almost like a regular set of panniers. Large outer pockets have ample space for shoes, a packed lunch, a jacket, or whatever else you need to carry.
The bag comes with a bright yellow rain cover that attaches to the outside of the bag. The nice part about it is that I can detach it and leave it at home on the days I know it won't rain. But if the forecast shows a chance of rain, I simply clip the rain cover on, just in case I need it.
The other thing I really like about the bag is that it comes with a shoulder strap. So when I park my bike, like on the days I take a shorter 5 km bike ride to the train, I simply unhook the bag from my bike, throw the strap over my shoulder, and, voila, a shoulder bag. Now I will admit, that the bag is a bit bulky to be an awesome shoulder bag. Thankfully, that's not it's primary purpose. The bag has to be that bulky to fit both the length and width of the clothes inside. The only downside is that I have to really squish it into the lockers at work (which I assume are standard size changing room lockers). Even an inch less width would make it a lot easier to fit.
Overall, I love this bag. I use it every single time I ride to work. The quality of it is top notch. It has big, sturdy zippers on it. And after almost a year's worth of daily use, it barely has any signs of wear at all. When it gets dusty from the road/trail, a quick rinse with the hose and it's cleaned right up.
Now if this is a bit too much for what you're looking for, Two Wheel Gear is close to releasing a smaller Pannier Backpack. I am extremely intrigued by this bag as well, especially for the days when I don't have as much to carry.
You really can't go wrong with either option!