Was just thinking of posting that a top case is the best thing in the world. I have an E52 that never leaves the bike. Also have a set of V35's in the mail, but will likely leave them at home day to day.
Standard contents are a Frogg Togg's rain suit, gym bag, iPad, sometimes my laptop bag, overpants with warm liners for my jacket and pants if the weather is potentially cool, and pair of shoes if needed.
It just adds so much versatility to the bike. Fantastic feeling knowing that you can go shopping and buy stuff, or leave things on the bike when needed with a reasonable sense of security, or just plain have the space to put stuff instead of having to carry it on you. Mine has carried a medium pizza a few times as well
I used to commute for a years with a backpack, once you try a top case, there's simply no going back. It's so convenient, you forget about just how ugly it is. Worst thing with a backpack are those super hot summer days, sitting in traffic, wearing gear, and on top of all that you have a heavy bag pressing on your back. With a top case, I even have somewhere to put my jacket if it gets too hot.
Was cleaning the bike yesterday and left the top case upstairs afterwards. Went for coffee in the evening and had to stuff my ipad in the back pocket of my jacket. Was thinking of riding to work this morning, but didn't want to carry my gym bag on my back, pack a pair of work shoes, or stuff my rain suit in it as they were calling for rain, ended up leaving the bike at home. Top case is going back on as soon as I get home.
If you use your bike as a mode of transportation and not just a toy, a top case is probably the best purchases you can make. Waterproof, secure, and if you're smart, always there. There's a reason why it's pretty much standard on every bike in europe