I've used both. Mostly because I encounter a deal that's too good to pass up, especially on assorted bits in a kit. But I always have OEM Dremel equivalents on hand. OEM is dependable and usually performs better. Plus I really like the Dremel EZ Lock.
But the ones I've decided to simply go with the cheap knock offs are the flappy wheel sanding drums and the wire wheels and brushes. I bought an assortment of steel wire wheels and brushes; 99 pcs for $20USD; $0.20 ea!
Then there's the set with copper wire wheels and brushes, along with three different grades of Scotch-brite knockoff material as polishing wheels and some 80 grit flappy sanding drums. 90 pcs for $16.99USD.
These items get the job done fine and service life indistinguishable from OEM... Maybe the wire wheels are rendered bare a little faster than OEM. Then again, maybe not? I bought an assortment of carbide bits for fine and rough shaping, but I haven't subjected them any torture tests yet.
The one accessory I'd highly recommend, especially with the wire wheels and cut-off disks would be the "dremel grinder cover" (search that on amazon). When you're cutting a lot with swarf or sparks flying, these keep it under control. It also let's me position the part and the tool in the best orientation without needing to manage where the debris flies.
Currently a set of 4 for about 11 or 12 bucks.
I love my Dremels. I've got three rotary tools, with the oldest going back to the early 1970s when my dad bought one, ostensibly for him, but I used it at least as much when building model cars. They didn't have built-in speed control back then so he rigged up a rheostat in a steel quad box at the end of a cord for a ghetto speed control. He used it mostly for drilling through PC boards which we made in-house.
My dad was a professional photographer, so we were making circuit layouts or copying them from the enthusiast mags of the day, like Popular Electronics, producing a mask using high contrast 4x5 litho film and then contact printing onto photoresist coated copper PC boards which we then etched. Little did I know that that was essentially what I'd be doing the next 40+ years! (I'm a microlithographer by profession
) Good times