Hello Andre,
I'm new to this forum, but found this topic since I want to build a new bike for commuting.
I was thinking of a combination of Rohloff hub and Bafang BBS02 mid drive kit.
Since you seem to have a similar combination: could you share your experiences? My local bike shop advised me against it. He said that shifting under load will be difficult. Even if I would stop to pedal for an instance, the BBS controller has some delay before the load is small enough to switch gear.
According to him, the commercial available ebikes with rohloff hub (with Bosh mid drives), have controllers which switch automatically the motor off when you switch gear, which should lead to a much smoother experience.
So I'm curious what your experience it 
Unless you're riding on hills, Jan, you don't
need a Rohloff. See
http://thorncyclesforum.co.uk/index.php?topic=7833.0 If you need or want a Rohloff and a midmounted motor (which is the best choice on all sorts of theoretical parameters), basically you can't choose a Thorn bike with a Rohloff because those adjust chain tension with an eccentric bottom bracket. My bike with the BBS motor is not a Thorn; another member with a BBS fitted it to derailleur Thorn, with a normal derailleur chain tension arm.
Your dealer is correct, a Bosch ebike with the Rolloff will be a smoother experience, and if you're merely buying/building a commuter for not overly demanding duty, it would be smart to go with the Bosch because it is likely to be the cheapest and least bothersome option. The good reasons for doing it the way we have is that you're already an expert bike builder, that you have a suitable frame already, that you intend to put the bike to heavier duty (speed, touring, load, really fat balloon tyres, rough roads) than the Bosch is designed for, and perhaps a few other reasons particular to individual cyclists; just to be different is not a good reason, and you won't save a huge amount of money.
If you're still determined to go ahead, my experience is that you can easily build a superior bike to the Bosch for any purpose but sheeplike commuting. It'll take a little more skill to ride, but it will be much more rewarding. My bike, compared to a Bosch, is the difference between a Bentley Continental with the biggest engine and a VW Beetle with the smallest engine they sell; my key interest is security of roadholding and handling at elevated speeds on very rough roads. See
http://thorncyclesforum.co.uk/index.php?topic=3798 To deal with the dealer's specific complaint about the Bafang BBS midmotors overrunning when you let the throttle go or stop pedaling while the pedelec facility is switched in, it's a genuine complaint on the first day or two that you have the motor fitted, and then you hardly notice it. That is because the Bafang mid motor has such good torque that you only use a few gears of the Rohloff in most circumstance and you easily "ride through" by giving more throttle or switching up a level of assistance (explained in a moment), so that the need for a gearchange is less. It's perhaps not scientific riding, but I was never a roadie, and so couldn't care less; power-riding does me fine. Also, I find that if I want to change gears upwards, I just do it with a very slight or no lift, and if I want to change gears downwards, there is usually enough momentum to lift off the throttle for enough time to do it. The Rohloff's rotary, sequential gear control is a big help, because you don't have to change one gear at a time, you can change as many as the rotation of your wrist allows, so that I often change five or six gears at once, brrrt. Sure, you can get caught out near the top of a very steep hill, but that is a sign that either your gears or your battery are badly matched to your motor; you wouldn't even have got that far that fast without the motor.
These midmotors have torque coming out of their ears; even the 250W has a better torque spread in use than the highly illegal BPM front motors of fond memory, the previous champions. There is a throttle which, if you buy your motor from the right dealer, is set to give maximum assistance at all levels and speed (i.e. you can go as fast as your gears and battery will permit and you wish to set the speed for in the settings of the controller -- note what it says about buying from the right dealer, because some limp wimps and self-declared community policemen sell crippled motors at whose electronic settings you can't get to undo their selfrighteous stupidity). There is also pedelec assistance, which depends on how fast you pedal and what gear you're in -- and again, if you buy from the right dealer, is set or can be set to the max your motor and battery combination is capable of.
This pedelec assistance is 9 levels, or you can set the switch to skip two at a time or three at a time to give you a more convenient 5 or three levels. I put the switches for all this right under my thumb and use it like the paddles under the wheel of my last Porsche to adjust my speed on the road, much more than I use the Rohloff gears. (In fact, I'm considering buying a battery so big that my Rohloff will be permanently in gear 11, the most efficient, direct gear, except for the very steepest hills, and except when used as an overdrive gearbox on some pieces of flat after a shallow hill where the motor will allow me to pedal up to full speed just at the top.) Below you can see the switch cluster peeking out from under my thumb at the bottom left of the photo. The reason you can't see the throttle is that my thumb is on it.
I have the 9 settings reduced to 5, of which no 3 is already far, far more powerful than my previous QSWXK front motor, itself no slouch, and four and five are like overdrives on a very powerful car, best saved for the open road. In town, on errands, I either have pedelec assistance switched off, and just use the throttle to break the bike loose, after that pedalling manually, or I use assistance level 1, which is plenty to let even a cardiac patient keep up with automobile traffic.
It could be said that the capabilities of the Bafang's built-in controller (again, if you buy from a dealer who sells a version that lets you get at the settings), used in the way I use them as a faux-tiptronic gearbox, makes the Rohloff an expensive indulgence. Of course, I could just be wearing the gears inside the Bafang midmotor faster; my QSWXK possibly (I haven't opened it yet) failed by melting the nylon gears.
There is no doubt in my mind that the Rohloff is sturdy enough to handle all the smaller Bafang motors (nominally 250 and 350W); I have no experience of the larger motors (500 and 750W).
I don't mean to be offensive, but just to ensure that we're on the same song sheet here, all of this is with hefty pedalling input from me; my bike is a key component in my exercise program so I use the motor to add a few watts when my heart is in danger of exceeding the permitted rate; in short, I control the motor's input by my heart rate as reported by my iPhone in the drybag on the photograph above (iPhone not in bag when photograph was taken because it was taking the photo, capice?). One could theoretically turn a pedal bike into an electric motorbike with one of the bigger Bafang motors and a big enough battery, but if that is what you want, it would be smarter and cheaper to buy an electric scooter.
***
I didn't actually look at the Bosch -- I was looking at a class of bike a few steps up and wouldn't ever consider a bike with frame dedicated to particular motor -- but my understanding is that it is the best middle-of-the-road readymade ebike out there, perfectly satisfactory to the majority of commuters. Building your own isn't difficult if you can read plain instructions and have ever swapped a bottom bracket, but in DIY there is always the risk of stuff
not working, and you having no recourse to exchange or warranty, and consequent waste of money.
Finally, you should consider Ian's point, that an electric motor setup is not a permanent capital asset but a consumable that you will write off totally in x number of years. I was happy when my QSWXK clocked out after three years; it cost me 16.5 eurocents per kilometer, which is exceedingly reasonable, so I bought another Bafang.
