I have seen that the Dremel brand bits usually work pretty well, and many times the generic ones are made so poorly they're not worth carrying off for free. You need to try one or two to see if they're a bargain or not.
I never had the digital-speed-controlled Dremel but I do have the next-one-down (XPR-400). I prefer the constantly-variable speed ones, and the speed controllers on the cheaper ones aren't nearly as nice as the 2nd-tier one.
By the by, I recently replaced the OEM bearings in my Dremel with some hybrid-ceramic bearings.
The results are mixed: a higher "low" speed and a lower "high" speed, but I made a couple errors that (I believe) degraded the motor's balance. You need a lathe and a milling machine to do it. There is a thread here discussing it-
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