Paweł Kadysz Like I said, I got used to the electronic viewfinder, the image stabilisation in my E-M5 is outstanding and lets me take really steady shots with lower shutter speeds. While shooting with Canon 50D anything above 800 ISO is just unusable. I got a bit lazy with my mirrorless. It's just much easier to take a technically good picture with it.
On the other hand, I browsed through my previous 365 and there are quite a few nice pics. It might be fun shooting with this old 50D again. You can see some of them here: privat.ly/page/9
Luke Stratta @Paweł, I use a DSLR nearly everyday to take photos, for both work and leisure, but haven't had the opportunity to spend any longer than a couple of minutes with a mirrorless.
Why would going back to a DSLR be potentially frustrating? Or are you just talking about using an older DSLR? (In that case, I know the pain, my Sony A100 is awful if there isn't midday summer sunlight on the subject...)