I have the DD 250 for small bits and the 750 for large.---The 250 won't do the double grind that the 750 will but I get really sharp bits from both.---Takes a little practice to get the hang of using them but it's worth it.---I'll pick a rainy day and set down and sharpen a fist full.
Stands to reason that bits may not cut quite as good as a new one because of the temper.---The temper quality probably falls off the further back you go on a bit.---The shank for instance is hardly tempered at all so the drill chuck can bite into it.---Otherwise with the slightest resistance on the bit from what your drilling, the bit would just spin in the chuck.---Take a 42% cobalt bit for instance, if the bit was tempered the same from end to end, the chuck couldn't begin to get a grip.
I have to also include the blessing of being able to hand sharpen bits on a grinder.---I use the side of the emery wheel because it is smoother.---This also takes some practice to perfect.---Helps if you can watch someone that knows how first.---A coworker of mine showed me how and for years it's all I had to keep my bits alive.---It paid off handsomely to, cause most people think when a bit is chipped or snapped in half it's ruined.---I would find them in the trash and put them on the grinder and,,,good as new.---I've found anything from an 1/8'' to 1 1/4 bits in the trash that just had a chunk chipped off one side and no one knew what to do for it but go get a new one.---Just a minute or two of flat grinding to get the cutting edges back to symmetrical and then start the roll.---Had to start taking them home to do it though cause when coworkers saw that the (broke-in-half bit) was good as new, it wasn't trash anymore.
The trick to hand sharpening is the pitch or angle you hold the bit to the emery, and the roll.---Got to keep them moving.---Stopping anywhere and you got a flat spot.---The angle has to be right and the grind has to have a roll or dome to it.---The high spot being the cutting edge naturally.---Right at the edge is the trickiest part, you have to look real close and make sure that the roll starts immediately after the cutting edge or it won't cut hot butter.
Seems to be no middle road for the DD.---Either you like them or you don't.